docs: what the harness now reaches, and the evidence for each assertion

The engine-coverage and measurement passages in context/code/clients.md, the
boundary table (four boundaries become six), and README's caveat -- which
named the grouped and multi-level DOM as never executed, and it now is. A fact
stated in more than one place is a grep, not an edit (gotcha 48).

context/test-evidence/clients.md records, per test, what fault was injected to
prove it can fail: the blanked help keys, the five grid-frame injections, the
404'd casts and the dead front-door link, and the mis-wired item-row click --
plus the two-command test-name diff that found that last one.

Playwright: 26 passed + 3 skipped, 34.4s / 33.7s (was 17 + 3, 15.1s); the
landing page's cast playback is most of the difference. node --test web/tests
26, cargo test --workspace 1394, both unchanged.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FC2jVn8Au7E9JPkw6bwXXH

README.md

@@ -429,10 +429,13 @@ binary whose job is a pivot table". If sharing a TUI-hosted document to a
 remote browser ever becomes a common case rather than an occasional one, this
 is worth revisiting on that evidence.
 
-Caveat, narrowed: `harness/` runs `web/app.js` under Chromium and `OpfsFs` under
-Chromium and Firefox, in CI. What is still never executed is `bindings.rs`, the
-grouped and multi-level DOM, the CSS and the `ResizeObserver` arithmetic — and
-Safari, whose Playwright build provides no OPFS at all.
+Caveat, narrowed twice: `harness/` runs `web/app.js` under Chromium — the
+standalone deployment under Firefox too — and `OpfsFs` under Chromium and
+Firefox, in CI. The grouped and multi-level DOM is executed since
+improvise-plcx, against a fixture document the harness serves beside the demo.
+What is still never executed is `bindings.rs`, the CSS and the
+`ResizeObserver` arithmetic — and Safari, whose Playwright build provides no
+OPFS at all.
 
 ## What's interesting about the architecture