Fix BAST deserialization bugs for LetStatement and MatchStatement

Two reader-side fixes that caused byte misalignment during BAST
deserialization, manifesting as failures on large models like
reactive-bbq (4428 nodes):

1. LetStatement: Added missing readU8() to consume the
   NODE_PATH_IDENTIFIER tag before readPathIdentifierNode() when
   reading the optional TypeRef added in 1.16.0. The writer uses
   writePathIdentifier() which includes the tag byte, but the
   reader was not consuming it — causing 1-byte drift.

2. MatchStatement: Changed reader to match the writer's byte
   layout where all case headers (loc + pattern + count) are
   written first, followed by all case items sequentially, then
   default items. The reader was incorrectly interleaving each
   case's header with its items.

Also removes pre-populated keywords from StringTable (90 entries
that were always serialized regardless of usage, ~5-85% size
reduction) and adds regression tests for epics and typed let
statements. Removes the reactive-bbq pending marker — all 187
riddl-models now pass round-trip testing.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: reid-spencer

commands/jvm/src/test/scala/com/ossuminc/riddl/commands/RiddlModelsRoundTripTest.scala

@@ -79,12 +79,7 @@ class RiddlModelsRoundTripTest
     "--show-usage-warnings=false"
   )
 
-  // Models with pre-existing validation errors in riddl-models
-  // that are unrelated to BAST round-trip correctness. These
-  // need fixes in the riddl-models repo, not here.
-  private val pendingModels: Set[String] = Set(
-    "hospitality/food-service/reactive-bbq" // validation errors in command handlers (missing event sends)
-  )
+  private val pendingModels: Set[String] = Set.empty
 
   "BAST round-trip" should {
     val confFiles = discoverModels(riddlModelsDir)

language/shared/src/main/scala/com/ossuminc/riddl/language/bast/BASTReader.scala

@@ -824,20 +824,42 @@ class BASTReader(bytes: Array[Byte])(using pc: PlatformContext) {
 
       case 11 => // Match
         val expression = readLiteralString()
+        // Writer writes all case headers (loc + pattern + count) first,
+        // then all case items sequentially, then default items.
         val numCases = reader.readVarInt()
-        val cases = (0 until numCases).map { _ =>
+        // Phase 1: Read all case headers (loc, pattern, count)
+        val caseHeaders = (0 until numCases).map { _ =>
           val caseLoc = readLocation()
           val pattern = readLiteralString()
-          val statements = readContentsDeferred[Statements]()
-          MatchCase(caseLoc, pattern, statements)
+          val count = reader.readVarInt()
+          (caseLoc, pattern, count)
         }.toSeq
-        val default = readContentsDeferred[Statements]()
-        MatchStatement(loc, expression, cases, default)
+        // Read default count
+        val defaultCount = reader.readVarInt()
+        // Phase 2: Read case items in order
+        val cases = caseHeaders.map { case (caseLoc, pattern, count) =>
+          val buffer = scala.collection.mutable.ArrayBuffer[Statements]()
+          var i = 0
+          while i < count do
+            buffer += readNode().asInstanceOf[Statements]
+            i += 1
+          end while
+          MatchCase(caseLoc, pattern, Contents(buffer.toSeq*))
+        }
+        // Phase 3: Read default items
+        val defaultBuffer = scala.collection.mutable.ArrayBuffer[Statements]()
+        var di = 0
+        while di < defaultCount do
+          defaultBuffer += readNode().asInstanceOf[Statements]
+          di += 1
+        end while
+        MatchStatement(loc, expression, cases, Contents(defaultBuffer.toSeq*))
 
       case 12 => // Let
         val identifier = readIdentifier()
         val hasTypeRef = reader.readU8() != 0
         val optTypeRef = if hasTypeRef then
+          reader.readU8() // consume NODE_PATH_IDENTIFIER tag
           val pid = readPathIdentifierNode()
           Some(TypeRef(loc, "type", pid))
         else None

language/shared/src/main/scala/com/ossuminc/riddl/language/bast/BASTWriter.scala

@@ -928,14 +928,14 @@ class BASTWriter(val writer: ByteBufferWriter, val stringTable: StringTable) {
     writer.writeU8(11) // Match statement
     writeLocation(s.loc)
     writeLiteralString(s.expression)
-    // Write cases
+    // Write all case headers (loc + pattern + count) first
     writer.writeVarInt(s.cases.size)
     s.cases.foreach { mc =>
       writeLocation(mc.loc)
       writeLiteralString(mc.pattern)
       writeContents(mc.statements)
     }
-    // Write default
+    // Write default count
     writeContents(s.default)
     // NOTE: case and default statement items are written by the Pass's traverse() override
   }

language/shared/src/main/scala/com/ossuminc/riddl/language/bast/StringTable.scala

@@ -11,50 +11,13 @@ import scala.collection.mutable
 /** String interning table for BAST serialization
   *
   * Provides efficient string storage by deduplicating common strings
-  * and allowing reference by index. Pre-populated with RIDDL keywords
-  * and predefined types to minimize file size.
+  * and allowing reference by index. Only strings actually used during
+  * serialization are stored — no pre-populated keywords.
   */
 class StringTable {
   private val strings = mutable.ArrayBuffer[String]()
   private val stringToIndex = mutable.HashMap[String, Int]()
 
-  // Pre-populate with RIDDL keywords for maximum compression
-  private val keywords = Seq(
-    // Structure keywords
-    "domain", "context", "entity", "adaptor", "saga", "streamlet",
-    "processor", "projector", "repository", "connector", "application",
-    "epic", "story", "useCase", "function", "type", "author", "user",
-    "term", "include", "import",
-
-    // Type keywords
-    "String", "Number", "Integer", "Decimal", "Boolean", "Date", "Time",
-    "DateTime", "TimeStamp", "URL", "UUID", "Current", "Id", "Abstract",
-    "Pattern", "Enumeration", "Alternation", "Aggregation", "Mapping",
-    "Graph", "Table", "Replica", "Reference", "Range", "Optional",
-    "ZeroOrMore", "OneOrMore", "required",
-
-    // Common structural keywords
-    "is", "as", "of", "from", "to", "by", "with", "for", "in", "on",
-    "at", "and", "or", "not", "if", "then", "else", "briefly", "described",
-    "explained", "contains", "yields", "returns", "requires", "ensures",
-    "invariant", "options", "pipe", "source", "sink", "flow", "merge",
-    "split", "void", "command", "event", "query", "result", "record",
-
-    // Handler keywords
-    "handler", "on", "other", "arbitrary",
-
-    // Relationship keywords
-    "sends", "acquires", "uses",
-
-    // Common names
-    "id", "name", "description", "value", "data", "message", "request",
-    "response", "error", "status", "code", "timestamp", "userId",
-    "created", "updated", "deleted"
-  )
-
-  // Initialize with keywords
-  keywords.foreach(intern)
-
   /** Intern a string and return its index
     *
     * If the string already exists in the table, returns the existing index.
@@ -119,25 +82,21 @@ class StringTable {
     }
   }
 
-  /** Clear the string table (except keywords) */
+  /** Clear the string table */
   def reset(): Unit = {
     strings.clear()
     stringToIndex.clear()
-    keywords.foreach(intern)
   }
 
   /** Statistics for debugging and optimization */
   def stats: StringTableStats = {
     val totalChars = strings.map(_.length).sum
     val avgChars = if strings.nonEmpty then totalChars.toDouble / strings.length else 0.0
-    val duplicatesAvoided = stringToIndex.size - keywords.size
-
     StringTableStats(
       totalStrings = strings.length,
       uniqueStrings = stringToIndex.size,
       totalCharacters = totalChars,
-      averageLength = avgChars,
-      duplicatesAvoided = duplicatesAvoided
+      averageLength = avgChars
     )
   }
 }
@@ -147,13 +106,12 @@ case class StringTableStats(
   totalStrings: Int,
   uniqueStrings: Int,
   totalCharacters: Int,
-  averageLength: Double,
-  duplicatesAvoided: Int
+  averageLength: Double
 )
 
 object StringTable {
 
-  /** Create a new empty string table with keywords pre-loaded */
+  /** Create a new empty string table */
   def apply(): StringTable = new StringTable()
 
   /** Deserialize a string table from a byte buffer
@@ -163,11 +121,6 @@ object StringTable {
     */
   def readFrom(reader: ByteBufferReader): StringTable = {
     val table = new StringTable()
-
-    // Clear the pre-populated keywords since we'll load from file
-    table.strings.clear()
-    table.stringToIndex.clear()
-
     val count = reader.readVarInt()
 
     var i = 0

passes/jvm/src/test/scala/com/ossuminc/riddl/passes/BASTIncrementalTest.scala

@@ -474,6 +474,37 @@ class BASTIncrementalTest extends AnyWordSpec with Matchers {
       }""")
     }
 
+    // Level 19.5: Test epic with user story and interactions
+    "handle epic with user story and interactions" in {
+      testRoundTrip("epic-test", """domain TestDomain is {
+        user Customer is "a person who orders food" with { briefly "customer" }
+        epic DineIn is {
+          user Customer wants "to order food" so that "they can eat"
+          case OrderFood is {
+            user Customer wants "to select items from the menu" so that "they can build an order"
+            step from user Customer "selects items from menu" to user Customer
+          }
+        } with { briefly "dining in" }
+      }""")
+    }
+
+    // Level 19.6: Test typed let statement (was the original LetStatement bug)
+    "handle typed let statement" in {
+      testRoundTrip("typed-let", """domain TestDomain is {
+        context Reservations is {
+          type ReservationBoardEntry is { name: String }
+          entity ReservationBoard is {
+            state main of ReservationBoard.ReservationBoardEntry
+            handler Input is {
+              on event TestDomain.Reservations.ReservationBoard.SomeEvent {
+                let entry: ReservationBoardEntry = "new ReservationBoardEntry from event"
+              }
+            }
+          }
+        }
+      }""")
+    }
+
     // Level 20: Test with rbbq-like structure
     "handle rbbq-like kitchen context" in {
       testRoundTrip("rbbq-kitchen", """domain ReactiveBBQ is {