#33: harden author-coloring PUA sentinels against intra-emphasis markdown

The F9/F10 author-coloring technique injects paired PUA sentinels into prose
source at span offsets, renders, then maps sentinels → cw-by-* spans. Two failure
modes when a span boundary met emphasis markup (characterized in session 0032):
- CASE1: a boundary strictly inside a delimiter run (a**b**c, between the two *)
  split `**`, breaking markdown-it's delimiter pairing (stray <em></em>, raw **).
- CASE3: a boundary inside an emphasis run (**bold** span covering **bo) rendered
  the emphasis but MISNESTED span/element (<strong>bo</span>ld</strong>).

Token-aware fix (both, per the issue #33 comment):
- injectSentinels: clamp any sentinel offset that lands strictly inside a
  delimiter run (* _ ~ `) to the run's start, so a sentinel never splits a run
  (CASE1). Delimiters are invisible once rendered, so this only shifts the colored
  boundary across markup. Drop spans that clamp to empty.
- sentinelsToSpans: replace the naive global split/join with a walker over the
  rendered HTML that emits the cw-by-* span only around TEXT runs — closing it
  before any <tag> and reopening after — so a span is always well-nested within
  inline elements (one span segment per text run, CASE3). Tags are copied
  verbatim with any stray sentinel stripped (no Private-Use-Area char leaks).

Pure, vscode-free, deterministic (INV-33). No regression: existing colorByAuthor /
renderReview cases stay green; the common no-emphasis case is byte-identical.

222 unit + 74/5 host E2E green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/trackChangesModel.ts

@@ -545,13 +545,33 @@ function authorBadge(authors: Set<AuthorKind>): { cls: string; label: string } |
     : { cls: "cw-by-human", label: "You" };
 }
 
+// Markdown emphasis / code delimiters whose RUNS must never be split by an
+// injected sentinel — a sentinel between two run chars (e.g. `*|*`) breaks
+// markdown-it's delimiter pairing (#33 CASE1).
+const isDelimChar = (c: string): boolean => c === "*" || c === "_" || c === "~" || c === "`";
+
+/**
+ * #33 (CASE1): a sentinel must not land STRICTLY INSIDE a delimiter run. If `at`
+ * sits between two identical delimiter chars, snap it to the run's start (a
+ * position outside the run) so the run stays intact. Delimiters are invisible once
+ * rendered, so snapping only shifts the colored boundary across markup, never over
+ * visible text.
+ */
+function clampOffDelimiterRun(raw: string, at: number): number {
+  if (at <= 0 || at >= raw.length) return at;
+  if (!(isDelimChar(raw[at]) && raw[at - 1] === raw[at])) return at;
+  let p = at;
+  while (p > 0 && raw[p - 1] === raw[at]) p--;
+  return p;
+}
+
 /** Inject paired sentinels into a prose block's raw text for the spans clipped to it. */
 function injectSentinels(raw: string, blockStart: number, spans: AuthorSpan[]): string {
   const inserts: { at: number; marker: string }[] = [];
   for (const s of spans) {
-    const lo = Math.max(0, s.start - blockStart);
-    const hi = Math.min(raw.length, s.end - blockStart);
-    if (hi <= lo) continue;
+    const lo = clampOffDelimiterRun(raw, Math.max(0, s.start - blockStart));
+    const hi = clampOffDelimiterRun(raw, Math.min(raw.length, s.end - blockStart));
+    if (hi <= lo) continue; // empty (or clamped to empty) span contributes nothing
     inserts.push({ at: lo, marker: SENT[s.author].open });
     inserts.push({ at: hi, marker: SENT[s.author].close });
   }
@@ -564,13 +584,69 @@ function injectSentinels(raw: string, blockStart: number, spans: AuthorSpan[]):
   return out;
 }
 
-/** Replace the rendered sentinels with author <span> tags. */
+const SENTINEL_OF: Record<string, { author: AuthorKind; open: boolean } | undefined> = {
+  [SENT.claude.open]: { author: "claude", open: true },
+  [SENT.claude.close]: { author: "claude", open: false },
+  [SENT.human.open]: { author: "human", open: true },
+  [SENT.human.close]: { author: "human", open: false },
+};
+const ALL_SENTINELS = new RegExp(
+  `[${SENT.claude.open}${SENT.claude.close}${SENT.human.open}${SENT.human.close}]`,
+  "g",
+);
+
+/**
+ * #33: token-aware replacement of the rendered author sentinels with `cw-by-*`
+ * spans. A naive global string-replace (the old approach) could emit a span that
+ * CROSSES an element boundary — `<span><strong>bo</span>ld</strong>` — when a
+ * boundary fell inside an emphasis run (CASE3). This walks the rendered HTML and
+ * emits the author span only around TEXT runs, CLOSING it before any `<tag>` and
+ * REOPENING it after, so a span is always well-nested within the inline elements
+ * (one `<span>` segment per text run). Tags are copied verbatim (with any stray
+ * sentinel stripped, so no Private-Use-Area char ever leaks). Pure, deterministic.
+ */
 function sentinelsToSpans(html: string): string {
-  return html
-    .split(SENT.claude.open).join('<span class="cw-by-claude">')
-    .split(SENT.claude.close).join("</span>")
-    .split(SENT.human.open).join('<span class="cw-by-human">')
-    .split(SENT.human.close).join("</span>");
+  const out: string[] = [];
+  let current: AuthorKind | null = null; // which author region we're inside
+  let spanOpen = false; // whether a <span> is currently open in `out`
+  const openSpan = () => {
+    if (current && !spanOpen) {
+      out.push(`<span class="cw-by-${current}">`);
+      spanOpen = true;
+    }
+  };
+  const closeSpan = () => {
+    if (spanOpen) {
+      out.push("</span>");
+      spanOpen = false;
+    }
+  };
+  for (let i = 0; i < html.length; i++) {
+    const ch = html[i];
+    const sentinel = SENTINEL_OF[ch];
+    if (sentinel) {
+      if (sentinel.open) current = sentinel.author; // span opens lazily before the next text char
+      else {
+        closeSpan();
+        current = null;
+      }
+      continue;
+    }
+    if (ch === "<") {
+      // An HTML tag: never let an author span straddle it (text `<` is escaped to
+      // &lt;, so a raw `<` is always a real tag). Copy the tag verbatim, sentinel-free.
+      closeSpan();
+      const gt = html.indexOf(">", i);
+      const end = gt === -1 ? html.length - 1 : gt;
+      out.push(html.slice(i, end + 1).replace(ALL_SENTINELS, ""));
+      i = end;
+      continue;
+    }
+    openSpan();
+    out.push(ch);
+  }
+  closeSpan();
+  return out.join("");
 }
 
 /**