MARS-419: browser-mode integration test for native-validation coalescing

Audit MARS-419 MEDIUM (BLOCK:11e985b). The jsdom unit suite synthesizes the DOM 'invalid' event and cannot reproduce real browser/base-ui microtask scheduling - exactly how the original 12-rows-for-3-submits bug escaped (unit passed, prod failed). Need a real-browser test (vitest browser mode @vitest/browser+playwright, OR standalone Playwright component test) mounting the production practica-form (or a representative base-ui Select-in-a-required-form), driving form.requestSubmit()/a real submit, asserting logFeedback fires exactly once per displayed validation block. Deferred as separate infra: adds a browser dependency + separate test project config + gitpush/CI wiring; harness choice (vitest-browser vs Playwright) wants PM/audit input. Deterministic jsdom regression (9 tests) shipped in ea73a24 and stays.