fix(typst): make HeadingTitle an external token to end LALR conflicts

Any item shared between headingTitleItem and document-level item causes
a shift/reduce conflict: the LALR automaton merges the two contexts and
makes the shared token ambiguous. The only structural fix is to make
HeadingTitle a terminal (external tokenizer) that reads greedily to EOL,
giving the LR state machine a context-isolated token that can never
collide with document-level item tokens.

Removes headingTitleItem sub-rule, HeadingText token, and updates
styleTags to match HeadingTitle directly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

services/web/frontend/js/features/source-editor/languages/typst/index.ts

@@ -30,8 +30,8 @@ export const TypstLanguage = LRLanguage.define({
         CodeArgs: foldInside,
       }),
       styleTags({
-        // Headings
-        'HeadingMark HeadingText': t.heading,
+        // Headings (HeadingTitle is an external token, one terminal per line)
+        'HeadingMark HeadingTitle': t.heading,
 
         // Comments
         'LineComment LineCommentContent': t.comment,

services/web/frontend/js/features/source-editor/lezer-typst/tokens.mjs

@@ -3,6 +3,7 @@
 import { ExternalTokenizer } from '@lezer/lr'
 import {
   HeadingMark,
+  HeadingTitle,
   RawBlockOpen,
   RawBlockBody,
   RawBlockClose,
@@ -47,6 +48,23 @@ export const headingTokenizer = new ExternalTokenizer(
   { contextual: false }
 )
 
+// ── headingTitleTokenizer ────────────────────────────────────────────────
+// Emits HeadingTitle — everything from the current position to end-of-line.
+// Using an external token (terminal) instead of grammar sub-items avoids
+// LALR(1) conflicts: any token shared between headingTitleItem and document-
+// level item causes the automaton to merge the two contexts.
+export const headingTitleTokenizer = new ExternalTokenizer(
+  (input, _stack) => {
+    let hasContent = false
+    while (input.next !== -1 && input.next !== NEWLINE) {
+      input.advance()
+      hasContent = true
+    }
+    if (hasContent) input.acceptToken(HeadingTitle)
+  },
+  { contextual: false }
+)
+
 // ── rawTokenizer ────────────────────────────────────────────────────────
 // Handles all three raw-block tokens (contextual: uses stack.canShift).
 //

services/web/frontend/js/features/source-editor/lezer-typst/typst.grammar

@@ -30,17 +30,12 @@ item {
 // ── Headings ──────────────────────────────────────────────────────────────
 // HeadingMark is produced by an external tokenizer that enforces the
 // start-of-line constraint and captures the "=+" prefix + trailing space.
-Heading { HeadingMark HeadingTitle }
-// Strong and Emphasis are intentionally excluded from headingTitleItem.
-// Including them causes an LALR(1) conflict: since Strong/Emphasis also appear
-// in document-level `item`, the LR automaton merges heading-title states with
-// document-item states, making "*" ambiguous (Strong opener vs. end of heading).
-// Instead, HeadingText is widened to consume "*" and "_" as plain text inside
-// headings — they are not interpreted as markup delimiters there.
-HeadingTitle { headingTitleItem+ }
-headingTitleItem {
-  CodeExpr | InlineMath | RawInline | Label | Ref | HeadingText
-}
+Heading { HeadingMark HeadingTitle? }
+// HeadingTitle is an external token (terminal) that reads everything to EOL.
+// Using an external token is the only reliable fix: any token that can start
+// both a headingTitleItem and a document-level item causes an LALR(1) conflict
+// after headingTitleItem+ because the automaton merges the two contexts.
+// External tokens are context-isolated by the LR state machine — no merging.
 
 // ── Comments ──────────────────────────────────────────────────────────────
 LineComment { "//" LineCommentContent }
@@ -131,6 +126,10 @@ Escape { "\\" EscapeChar }
   HeadingMark
 }
 
+@external tokens headingTitleTokenizer from "./tokens.mjs" {
+  HeadingTitle
+}
+
 @external tokens rawTokenizer from "./tokens.mjs" {
   RawBlockOpen,
   RawBlockBody,
@@ -188,8 +187,8 @@ Escape { "\\" EscapeChar }
   // Math content — everything between the $ delimiters (no crossing newlines).
   MathContent { ![$\n]+ }
 
-  // Text tokens for different markup contexts; each excludes its own delimiters.
-  HeadingText  { ![\n$#`<@\\]+  }
+  // Text tokens for markup contexts; each excludes its own delimiters.
+  // HeadingText is gone: HeadingTitle is now an external token (see above).
   StrongText   { ![\n*$#`@\\]+    }
   EmphText     { ![\n_$#`@\\]+    }