Make BASTLoader JS-compatible with platform-specific implementation

- Created BASTLoaderPlatform in jvm-native/ with blocking Await.result()
- Created BASTLoaderPlatform in js/ returning error (no file I/O in browser)
- Updated BASTLoader to delegate to platform-specific implementation
- All 6 SharedBASTTest tests now pass on JVM, JS, and Native

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: reidspencer

NOTEBOOK.md

@@ -102,10 +102,10 @@ Benchmark on `dokn.riddl` (7.5KB, 167 nodes):
 |----------|--------|-------|
 | JVM | ✅ Passing | 6 tests in SharedBASTTest |
 | Native | ✅ Passing | 6 tests in SharedBASTTest |
-| JS | ⚠️ Known limitation | No local file I/O on browser platform |
+| JS | ✅ Passing | 6 tests in SharedBASTTest |
 
-**Note**: JS cannot support BAST file loading because `DOMPlatformContext` throws `FileNotFoundException`
-for `file://` URLs. This is an inherent platform limitation, not a bug. BAST import loading is a JVM/Native feature.
+**Note**: BAST serialization/deserialization works on all platforms. BAST *file import loading* is
+JVM/Native only (JS returns error message since browser can't do local file I/O).
 
 ### Next Steps
 
@@ -140,13 +140,13 @@ for `file://` URLs. This is an inherent platform limitation, not a bug. BAST imp
 **Completed**:
 - Created `SharedBASTTest.scala` in `passes/shared/src/test/` with 6 tests
 - Tests build AST programmatically (avoid parser's BAST import loading)
-- JVM: All 6 tests pass ✅
-- Native: All 6 tests pass ✅
-- JS: Known platform limitation (no local file I/O)
+- Made `BASTLoader` platform-aware with `BASTLoaderPlatform`:
+  - JVM/Native: Uses blocking `Await.result()` for file loading
+  - JS: Returns error message (file I/O not supported)
+- All platforms pass: JVM ✅, Native ✅, JS ✅
 
-**Conclusion**: BAST serialization/deserialization works correctly on JVM and Native.
-JS cannot support BAST file loading because `DOMPlatformContext` throws `FileNotFoundException`
-for `file://` URLs - this is an inherent browser platform limitation, not a bug.
+**Key insight**: Separated blocking I/O code into `BASTLoaderPlatform` (in `jvm-native/` and `js/`)
+to allow JS linker to succeed while maintaining full functionality on JVM/Native.
 
 ### January 16, 2026 (Cleanup & Benchmarking)
 

language/js/src/main/scala/com/ossuminc/riddl/language/bast/BASTLoaderPlatform.scala

@@ -0,0 +1,26 @@
+/*
+ * Copyright 2019-2026 Ossum, Inc.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package com.ossuminc.riddl.language.bast
+
+import com.ossuminc.riddl.language.AST.*
+import com.ossuminc.riddl.utils.{PlatformContext, URL}
+
+/** JavaScript implementation of BAST loading - throws exception since file I/O is not supported. */
+private[bast] object BASTLoaderPlatform {
+
+  /** BAST import loading is not supported on JavaScript platform.
+    *
+    * The browser environment cannot perform local file I/O operations required
+    * to load BAST files. This is an inherent platform limitation.
+    *
+    * @throws UnsupportedOperationException always
+    */
+  def loadSingleImport(bi: BASTImport, baseURL: URL)(using pc: PlatformContext): Either[String, Nebula] = {
+    Left("BAST import loading is not supported on JavaScript platform. " +
+         "The browser cannot perform local file I/O operations.")
+  }
+}

language/jvm-native/src/main/scala/com/ossuminc/riddl/language/bast/BASTLoaderPlatform.scala

@@ -0,0 +1,53 @@
+/*
+ * Copyright 2019-2026 Ossum, Inc.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package com.ossuminc.riddl.language.bast
+
+import com.ossuminc.riddl.language.AST.*
+import com.ossuminc.riddl.language.Messages
+import com.ossuminc.riddl.utils.{PlatformContext, URL}
+
+import scala.concurrent.{Await, ExecutionContext}
+import scala.concurrent.duration.*
+import scala.util.{Failure, Success, Try}
+
+/** JVM/Native implementation of BAST loading with blocking I/O support. */
+private[bast] object BASTLoaderPlatform {
+
+  /** Load a single BAST import using blocking I/O.
+    *
+    * @param bi The BASTImport to load
+    * @param baseURL The base URL for resolving relative paths
+    * @param pc The platform context
+    * @return Either an error message or the loaded Nebula
+    */
+  def loadSingleImport(bi: BASTImport, baseURL: URL)(using pc: PlatformContext): Either[String, Nebula] = {
+    Try {
+      // Resolve the path relative to the base URL
+      val bastURL = if URL.isValid(bi.path.s) then {
+        URL(bi.path.s)
+      } else {
+        baseURL.parent.resolve(bi.path.s)
+      }
+
+      // Load the BAST file bytes
+      implicit val ec: ExecutionContext = pc.ec
+      val future = pc.load(bastURL).map { data =>
+        // Parse as BAST - note: data is loaded as String, convert to bytes
+        val bytes = data.getBytes("ISO-8859-1") // Binary data preserved
+        val reader = BASTReader(bytes)
+        reader.read() // Returns Either[Messages, Nebula]
+      }
+
+      // Wait for the result (with timeout) - blocking I/O
+      Await.result(future, 30.seconds)
+    } match {
+      case Success(Right(nebula)) => Right(nebula)
+      case Success(Left(msgs)) => Left(msgs.map(_.format).mkString("; "))
+      case Failure(ex) => Left(ex.getMessage)
+    }
+  }
+}

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

@@ -12,9 +12,6 @@ import com.ossuminc.riddl.language.Messages.Messages
 import com.ossuminc.riddl.utils.{PlatformContext, URL}
 
 import scala.collection.mutable
-import scala.concurrent.{Await, ExecutionContext}
-import scala.concurrent.duration.*
-import scala.util.{Failure, Success, Try}
 
 /** Utility for loading BAST imports.
   *
@@ -78,37 +75,17 @@ object BASTLoader {
   }
 
   /** Load a single BAST import.
+    *
+    * This delegates to the platform-specific implementation since JVM/Native
+    * support blocking I/O while JavaScript does not.
     *
     * @param bi The BASTImport to load
     * @param baseURL The base URL for resolving relative paths
     * @param pc The platform context
     * @return Either an error message or the loaded Nebula
     */
   private def loadSingleImport(bi: BASTImport, baseURL: URL)(using pc: PlatformContext): Either[String, Nebula] = {
-    Try {
-      // Resolve the path relative to the base URL
-      val bastURL = if URL.isValid(bi.path.s) then {
-        URL(bi.path.s)
-      } else {
-        baseURL.parent.resolve(bi.path.s)
-      }
-
-      // Load the BAST file bytes
-      implicit val ec: ExecutionContext = pc.ec
-      val future = pc.load(bastURL).map { data =>
-        // Parse as BAST - note: data is loaded as String, convert to bytes
-        val bytes = data.getBytes("ISO-8859-1") // Binary data preserved
-        val reader = BASTReader(bytes)
-        reader.read() // Returns Either[Messages, Nebula]
-      }
-
-      // Wait for the result (with timeout)
-      Await.result(future, 30.seconds)
-    } match {
-      case Success(Right(nebula)) => Right(nebula)
-      case Success(Left(msgs)) => Left(msgs.map(_.format).mkString("; "))
-      case Failure(ex) => Left(ex.getMessage)
-    }
+    BASTLoaderPlatform.loadSingleImport(bi, baseURL)
   }
 
   /** Check if a Root has any unloaded BASTImport nodes.