fix(typst): eliminate LALR(1) conflict on heading title items

Removing Strong and Emphasis from headingTitleItem eliminates the
conflict: both appear in document-level item, causing the LR automaton
to merge heading-title states with document-item states and make "*"
ambiguous (Strong opener vs. end of heading).

HeadingText is widened to ![\n$#`<@\\]+ so "*" and "_" inside headings
are consumed as plain text rather than producing error nodes.

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

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

@@ -31,12 +31,15 @@ item {
 // HeadingMark is produced by an external tokenizer that enforces the
 // start-of-line constraint and captures the "=+" prefix + trailing space.
 Heading { HeadingMark HeadingTitle }
-// headingTitleItem+ (not *) so HeadingTitle is never empty: the generator
-// would otherwise produce a shift/reduce conflict on "*" after HeadingMark,
-// unable to tell if "*" starts a Strong inside the heading or at doc level.
+// 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 {
-  Strong | Emphasis | CodeExpr | InlineMath | RawInline | Label | Ref | HeadingText
+  CodeExpr | InlineMath | RawInline | Label | Ref | HeadingText
 }
 
 // ── Comments ──────────────────────────────────────────────────────────────
@@ -186,7 +189,7 @@ Escape { "\\" EscapeChar }
   MathContent { ![$\n]+ }
 
   // Text tokens for different markup contexts; each excludes its own delimiters.
-  HeadingText  { ![\n*_$#`<@\\]+  }
+  HeadingText  { ![\n$#`<@\\]+  }
   StrongText   { ![\n*$#`@\\]+    }
   EmphText     { ![\n_$#`@\\]+    }