perf: fix top-5 hot-path allocation anti-patterns

1. lsp/completion: add zero-alloc hasPrefixFold helper and use it in
   anchorItems, refLabelItems, kindItems, directivePathItems — removes
   one strings.ToLower allocation per symbol/kind/file on every LSP
   completion request.

2. lsp/completion: return nil (not []completionItem{}) from helper
   early-exits; outer handleCompletion already converts nil → [].

3. lsp/symbols: return nil (not []documentSymbol{}) from buildOutline
   when index has no file entry.

4. rules/requiredstructure: compile buildFieldPattern once per body
   line instead of once per field on that line, eliminating redundant
   regexp.MustCompile calls when a line contains multiple {field}
   references.

5. rules/concisenessscoring/classifier: return nil (not []string{})
   from dedupeSorted on empty input, per project convention.

https://claude.ai/code/session_01EekcsPRZ89tht6Fsh3y4d8

Author: claude

internal/lsp/completion.go

@@ -52,7 +52,7 @@ func (s *Server) completionItems(ctx index.CompletionContext, rel string, idx *i
 		return s.anchorItems(rel, ctx.Prefix, idx, true)
 	case index.CompletionAnchorOtherFile:
 		if ctx.TargetFile == "" {
-			return []completionItem{}
+			return nil
 		}
 		return s.anchorItems(ctx.TargetFile, ctx.Prefix, idx, false)
 	case index.CompletionRefLabel:
@@ -62,7 +62,16 @@ func (s *Server) completionItems(ctx index.CompletionContext, rel string, idx *i
 	case index.CompletionDirectivePath:
 		return s.directivePathItems(rel, ctx.Prefix, idx)
 	}
-	return []completionItem{}
+	return nil
+}
+
+// hasPrefixFold reports whether s begins with prefixLower (already lowercased),
+// case-insensitively and without allocating.
+func hasPrefixFold(s, prefixLower string) bool {
+	if len(s) < len(prefixLower) {
+		return false
+	}
+	return strings.EqualFold(s[:len(prefixLower)], prefixLower)
 }
 
 // anchorItems returns completion items for heading anchors in the given file.
@@ -71,15 +80,15 @@ func (s *Server) completionItems(ctx index.CompletionContext, rel string, idx *i
 func (s *Server) anchorItems(file, prefix string, idx *index.Index, sameFile bool) []completionItem {
 	fe, ok := idx.File(file)
 	if !ok {
-		return []completionItem{}
+		return nil
 	}
 	prefixLower := strings.ToLower(prefix)
 	var items []completionItem
 	for _, sym := range fe.Symbols {
 		if sym.Kind != index.SymbolHeading || sym.Anchor == "" {
 			continue
 		}
-		if !strings.HasPrefix(strings.ToLower(sym.Anchor), prefixLower) {
+		if !hasPrefixFold(sym.Anchor, prefixLower) {
 			continue
 		}
 		sortPfx := "b"
@@ -103,15 +112,15 @@ func (s *Server) anchorItems(file, prefix string, idx *index.Index, sameFile boo
 func (s *Server) refLabelItems(file, prefix string, idx *index.Index) []completionItem {
 	fe, ok := idx.File(file)
 	if !ok {
-		return []completionItem{}
+		return nil
 	}
 	prefixLower := strings.ToLower(prefix)
 	var items []completionItem
 	for _, sym := range fe.Symbols {
 		if sym.Kind != index.SymbolLinkRef {
 			continue
 		}
-		if !strings.HasPrefix(strings.ToLower(sym.Anchor), prefixLower) {
+		if !hasPrefixFold(sym.Anchor, prefixLower) {
 			continue
 		}
 		items = append(items, completionItem{
@@ -128,12 +137,12 @@ func (s *Server) refLabelItems(file, prefix string, idx *index.Index) []completi
 func (s *Server) kindItems(prefix string) []completionItem {
 	cfg, _, _ := s.snapshotConfig()
 	if cfg == nil {
-		return []completionItem{}
+		return nil
 	}
 	prefixLower := strings.ToLower(prefix)
 	var items []completionItem
 	for k := range cfg.Kinds {
-		if !strings.HasPrefix(strings.ToLower(k), prefixLower) {
+		if !hasPrefixFold(k, prefixLower) {
 			continue
 		}
 		items = append(items, completionItem{
@@ -168,7 +177,7 @@ func (s *Server) directivePathItems(rel, prefix string, idx *index.Index) []comp
 	var items []completionItem
 	for _, f := range files {
 		relF := relFromDir(dir, f)
-		if !strings.HasPrefix(strings.ToLower(relF), prefixLower) {
+		if !hasPrefixFold(relF, prefixLower) {
 			continue
 		}
 		label := relF

internal/lsp/symbols.go

@@ -459,7 +459,7 @@ func buildOutline(source []byte) []documentSymbol {
 	idx.Update("buffer", source)
 	fe, ok := idx.File("buffer")
 	if !ok {
-		return []documentSymbol{}
+		return nil
 	}
 
 	var fmKids []documentSymbol

internal/rules/concisenessscoring/classifier/model.go

@@ -470,7 +470,7 @@ func phraseMarker(phrase string) string {
 
 func dedupeSorted(values []string) []string {
 	if len(values) == 0 {
-		return []string{}
+		return nil
 	}
 	seen := map[string]struct{}{}
 	out := make([]string, 0, len(values))

internal/rules/requiredstructure/rule.go

@@ -1332,11 +1332,14 @@ func collectBodySyncPoints(
 		}
 		if currentHeading >= 0 && trimmed != "" {
 			fields := fieldinterp.Fields(trimmed)
-			for _, f := range fields {
-				syncPoints[currentHeading] = append(
-					syncPoints[currentHeading],
-					syncPoint{Field: f, InBody: true, BodyText: trimmed, compiled: buildFieldPattern(trimmed)},
-				)
+			if len(fields) > 0 {
+				compiled := buildFieldPattern(trimmed)
+				for _, f := range fields {
+					syncPoints[currentHeading] = append(
+						syncPoints[currentHeading],
+						syncPoint{Field: f, InBody: true, BodyText: trimmed, compiled: compiled},
+					)
+				}
 			}
 		}
 	}