fix(typst): guard RawBlockBody against LALR-merged body-text states

canShift(RawBlockBody) returns true in states LALR-merged with the
post-RawBlockOpen state, causing the tokenizer to consume all remaining
body text as one giant RawBlockBody. Add a backward character scan:
require newline immediately before input.pos, then walk back past any
lang tag (A-Za-z0-9) and verify three backticks precede it. Body-text
positions never have backtick-backtick-backtick there, so the guard
rejects them.

This was the root cause of everything after the first heading being
black: RawBlockBody swallowed the entire document from the user-name
line onward, making headings, bold, italic and math invisible.

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

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

@@ -105,6 +105,20 @@ export const rawTokenizer = new ExternalTokenizer(
     }
 
     if (stack.canShift(RawBlockBody)) {
+      // Guard: must genuinely follow a RawBlockOpen (which ends with \n).
+      // Walk backward past any lang tag (A-Za-z0-9) and require ```.
+      // This blocks spurious LALR-merged states from consuming body text.
+      if (input.peek(-1) !== NEWLINE) return
+      let back = -2
+      while (
+        (input.peek(back) >= 65 && input.peek(back) <= 90) ||
+        (input.peek(back) >= 97 && input.peek(back) <= 122) ||
+        (input.peek(back) >= 48 && input.peek(back) <= 57)
+      ) { back-- }
+      if (input.peek(back)     !== BACKTICK ||
+          input.peek(back - 1) !== BACKTICK ||
+          input.peek(back - 2) !== BACKTICK) return
+
       let hasContent = false
       while (input.next !== -1) {
         if (