fix(typst): restore HeadingTitle token to fix broken syntax highlighting

Removing HeadingTitle from the grammar left HeadingTitle? undeclared,
causing the Lezer grammar compiler to fail and producing no parser
output — hence everything rendered as unstyled black text.

Dual approach to prevent heading style bleed:
- HeadingTitle exists in grammar with contextual: true + canShift guard
  (prevents it from matching in body-text LALR states)
- HeadingTitle is intentionally absent from styleTags so even spurious
  matches cannot apply heading colour to body text
- ViewPlugin styles heading titles by finding HeadingMark nodes and
  extending tok-heading decoration to end-of-line

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

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

@@ -110,12 +110,12 @@ const typstHighlightStyle = HighlightStyle.define([
   { tag: t.emphasis, fontStyle: 'italic' },
 ])
 
-// Heading title decoration: the grammar only has a HeadingMark token (the "=+ "
-// prefix). This plugin walks the syntax tree, finds each HeadingMark, and
-// extends a heading-style decoration to the end of the line so the title text
-// gets the same blue + bold treatment without needing a HeadingTitle token
-// (which had LALR state-merging issues causing body text to bleed into heading
-// style in the previous implementation).
+// Heading title decoration: HeadingTitle exists in the grammar but is NOT in
+// styleTags, so it never applies heading colour on its own. This plugin walks
+// the syntax tree, finds each HeadingMark, and extends a heading-style
+// decoration to end-of-line. Belt-and-suspenders: even if HeadingTitle fires
+// spuriously (LALR state merging), no bleed happens because styleTags ignores
+// it; and the ViewPlugin only decorates lines that start with a HeadingMark.
 const headingTitleMark = Decoration.mark({
   class: 'tok-heading',
   attributes: { style: 'font-weight:bold' },

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,
@@ -50,6 +51,28 @@ export const headingTokenizer = new ExternalTokenizer(
   { contextual: false }
 )
 
+// ── headingTitleTokenizer ───────────────────────────────────────────────
+// Emits HeadingTitle — the rest of the line after HeadingMark.
+// contextual: true + canShift ensures this only fires when the parser is
+// in the state immediately after accepting HeadingMark, preventing the
+// token from being matched in body-text states due to LALR state merging.
+// HeadingTitle is intentionally absent from styleTags; the ViewPlugin in
+// index.ts decorates heading title text by extending from HeadingMark.to
+// to line end, which means even spurious HeadingTitle tokens (if the
+// contextual guard ever fails) cannot bleed heading style into body text.
+export const headingTitleTokenizer = new ExternalTokenizer(
+  (input, stack) => {
+    if (!stack.canShift(HeadingTitle)) return
+    let hasContent = false
+    while (input.next !== -1 && input.next !== NEWLINE) {
+      input.advance()
+      hasContent = true
+    }
+    if (hasContent) input.acceptToken(HeadingTitle)
+  },
+  { contextual: true }
+)
+
 // ── rawTokenizer ────────────────────────────────────────────────────────
 // Handles all three raw-block tokens (contextual: uses stack.canShift).
 //

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

@@ -6,8 +6,7 @@
 //   rawInlineTokenizer    — single-backtick raw inline content
 //   codeBlockTokenizer    — brace-depth tracking inside #{ ... }
 //   blockCommentTokenizer — depth-tracked nested /* ... */ comments
-// Note: heading title text is NOT a grammar token. The ViewPlugin in
-// languages/typst/index.ts decorates heading lines via the syntax tree.
+// headingTitleTokenizer — the heading title text (rest of line after HeadingMark)
 
 @top Document { item* }
 
@@ -32,9 +31,9 @@ item {
 // ── Headings ──────────────────────────────────────────────────────────────
 // HeadingMark is produced by an external tokenizer that enforces the
 // start-of-line constraint and captures the "=+" prefix + trailing space.
-// The heading title (text after the marker) is NOT a grammar token; a
-// ViewPlugin in index.ts decorates the rest of the line via the syntax tree.
-Heading { HeadingMark }
+// HeadingTitle is an external token that reads the rest of the line to EOL,
+// avoiding LALR(1) conflicts with document-level items.
+Heading { HeadingMark HeadingTitle? }
 
 // ── Comments ──────────────────────────────────────────────────────────────
 LineComment { "//" LineCommentContent? }
@@ -128,6 +127,10 @@ Escape { "\\" EscapeChar }
   HeadingMark
 }
 
+@external tokens headingTitleTokenizer from "./tokens.mjs" {
+  HeadingTitle
+}
+
 @external tokens rawTokenizer from "./tokens.mjs" {
   RawBlockOpen,
   RawBlockBody,