#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>

test/trackChangesModel.test.ts

@@ -168,6 +168,45 @@ describe("colorByAuthor", () => {
     expect(html).toContain('<span class="cw-by-human">hello</span>');
     expect(html).toContain("world");
   });
+
+  // #33: author-coloring must be sentinel-safe around markdown emphasis. These
+  // exercise the REAL markdown-it renderer (via renderReview on an unchanged doc),
+  // since the failure is in markdown-it's inline parsing / element nesting.
+  // CASE1 — a span boundary strictly inside a delimiter run must not split it.
+  test("#33 CASE1: a span boundary inside ** does not break emphasis parsing", () => {
+    const doc = "a**b**c";
+    const html = renderReview(doc, doc, [{ start: 0, end: 2, author: "human" }], []);
+    expect(html).toContain("<strong>b</strong>"); // emphasis still renders
+    expect(html).not.toContain("**"); // no raw delimiters left
+    expect(html).not.toContain("<em></em>"); // no stray empty emphasis (the parse-break symptom)
+    expect(html).toContain('class="cw-by-human"'); // coloring present
+  });
+  // CASE3 — a span boundary inside an emphasis run must color the text without
+  // misnesting span/element (the span is split at the element boundary).
+  test("#33 CASE3: a span boundary inside **bold** colors the text without misnesting", () => {
+    const doc = "**bold**";
+    const html = renderReview(doc, doc, [{ start: 0, end: 4, author: "human" }], []); // covers "**bo"
+    expect(html).toContain("<strong>");
+    expect(html).toContain('<span class="cw-by-human">bo</span>ld</strong>'); // span INSIDE strong, closed before "ld"
+    expect(html).not.toContain('cw-by-human"><strong>'); // NOT the old misnest (span wrapping the <strong> open)
+  });
+  // CASE2 — a span covering a whole emphasis run stays correct (regression).
+  test("#33 CASE2: a span over the whole **bold** colors it correctly", () => {
+    const doc = "**bold**";
+    const html = renderReview(doc, doc, [{ start: 0, end: 8, author: "human" }], []);
+    expect(html).toContain("<strong>");
+    expect((html.match(/cw-by-human/g) ?? []).length).toBe(1);
+    expect(html).toContain("bold");
+  });
+  test("#33: no Private-Use-Area sentinel chars leak into the rendered output", () => {
+    const doc = "a**b**c and `co de` and _x_";
+    const spans: AuthorSpan[] = [
+      { start: 0, end: 2, author: "human" },
+      { start: 12, end: 16, author: "claude" },
+    ];
+    const html = renderReview(doc, doc, spans, []);
+    expect(html).not.toMatch(/[\uE000-\uF8FF]/); // no leftover BMP Private-Use-Area sentinels
+  });
 });
 
 import { renderPlain } from "../src/trackChangesModel";