frontend/tests/browser.rs wasm suite not wired into CI; 21 pre-existing failures masked #20

Summary: The full frontend/tests/browser.rs wasm-bindgen-test
suite (headless-browser DOM tests) is never run by CI —
.github/workflows/wasm-tests.yml only runs -- --lib and
-- --test collab_e2e. Running the suite directly shows 127
passed / 21 failed, all 21 pre-existing at the merge-base, not
introduced by the syntax-highlighting branch. Because this suite
never runs in CI, it did not catch a real regression on this
branch until a manual full run was performed.

Severity: S2 — a real regression (two browser tests broken by
the highlighting DOM change) shipped past CI unnoticed; the test
suite exists but provides no actual safety net for a large class
of DOM-observable behavior (headings, lists, code blocks, paste).

Environment:

• Deploy: local (headless Firefox, wasm-bindgen-test-runner)
• Commit: branch head of code-blocks-syntax-highlighting;
  merge-base 3546f0c
• Browser: headless Firefox (headless Chrome crashed with SIGKILL
  in this sandbox; not evaluated further)

Repro steps:

1. cd frontend
2. Run the full browser integration target under
   wasm-pack test --headless --firefox -- --test browser
   (or the CI-equivalent invocation used for collab_e2e, swapped
   to --test browser).
3. Observe the pass/fail summary.

Expected: All tests in frontend/tests/browser.rs pass, or —
if any are legitimately quarantined — they are marked #[ignore]
with a tracked reason, not silently absent from CI.

Observed: 127 passed / 21 failed. Checking out the merge-base
commit (3546f0c) and re-running reproduces the same 21 failures,
confirming they are pre-existing and unrelated to the highlighting
branch's changes — but nothing catches them today because the
target isn't in any CI workflow.

Breakdown of the 21:

• 15 bare-tag assertion failures — tests assert on raw HTML
  tags (e.g. html.contains("<pre>")-style), but every block now
  carries a data-block-id attribute stamped by
  EditorState/Node::needs_block_id in frontend/src/editor/model.rs,
  which has been present since the initial public release
  (ec2d37d). Example: renders_heading in
  frontend/tests/browser.rs.
• 6 clipboard-simulation harness-gap failures — tests
  construct a synthetic ClipboardEvent/DataTransfer and dispatch
  it, but headless Firefox does not read the synthetic data back
  through the production paste path, which logs
  "clipboard empty — nothing to paste" (see the
  debug::warn("paste", ...) call in frontend/src/editor/view.rs).
  Affected tests (by name, in frontend/tests/browser.rs):
  paste_markdown_heading_creates_h1_in_dom,
  paste_markdown_bullet_list_creates_ul_in_dom,
  paste_pre_wrapped_markdown_with_text_plain_parses_as_markdown,
  paste_pre_wrapped_markdown_without_text_plain_parses_as_markdown,
  paste_pre_code_without_language_preserves_codeblock,
  paste_real_code_block_preserves_codeblock.

Evidence:

• Full-suite run at branch head: 127 passed / 21 failed (headless
  Firefox).
• Same 21 failures reproduced at merge-base 3546f0c, confirming
  pre-existing status.
• Chrome headless: process SIGKILL'd in this sandbox environment;
  not further diagnosed — may be sandbox-specific rather than a
  browser-target bug.

Suggested approach:

1. Fix the 15 bare-tag assertions to account for data-block-id
   (e.g. assert on tag + attribute presence rather than an exact
   substring, or scope the substring check past the attribute).
2. Decide on a harness approach for the 6 paste tests — either get
   the synthetic clipboard event readable in headless Firefox, or
   explicitly #[ignore = "..."] them with a tracked follow-up
   reason.
3. Add --test browser to the wasm-pack invocation in
   .github/workflows/wasm-tests.yml (alongside the existing
   --lib and --test collab_e2e targets) once (1) and (2) are
   resolved, so the suite actually gates merges going forward.

Suspected area:

• .github/workflows/wasm-tests.yml (CI wiring gap)
• frontend/src/editor/model.rs — Node::needs_block_id (source of
  the data-block-id stamping that the 15 assertions don't account
  for)
• frontend/src/editor/view.rs — paste-empty-clipboard log path
• frontend/tests/browser.rs — the 21 affected test bodies