Store each action screenshot once in the trace zip - #508

What & why

Closes #495. Part of #494. Stacked on #507.

Every per-action frame-snapshot event carried its screenshot again as base64, inside an HTML page wrapped around an <img>, while the same screenshot was already stored under resources/. On the WDIO cucumber example trace that was 529 KB of trace.trace's 538 KB zipped. The snapshot's <img> now points at a URL that the snapshot's resourceOverrides maps to the stored file, so each screenshot is stored once.

Type of change

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Polish (an improvement to an existing feature)
• Breaking change (existing behavior changes for users)
• Documentation
• Internal (build, CI, dependencies, tooling)

Packages touched

• shared (types and contracts)
• core (framework-agnostic capture/reporting)
• elements (published element/snapshot API — @wdio/elements)
• service (WebdriverIO adapter)
• nightwatch-devtools (Nightwatch adapter)
• selenium-devtools (Selenium adapter)
• selenium-devtools-py (Selenium Python adapter)
• backend (server)
• app (UI)
• script (page-injected runtime)
• trace (Trace mode)

Capability matrix

• packages/conformance/src/features.ts updated, and tables regenerated with pnpm docs:capabilities, if this adds or changes user-facing behaviour

Not needed: traces open and replay as before; only their size changes.

Notes for reviewers

Measured

Rebuilt from the same capture with the old and new code; the old-code rebuild was byte-identical to the original zip.

Design notes

• The <img> points at https://screenshot.wdio-devtools.invalid/action.<png|jpeg>. One function now names the screenshot file for the writer, the filmstrip and the override, so they cannot drift apart.
• The frame-snapshots exist for other viewers of the format; our player replays from the mutation stream and never read them. Playwright's viewer (which Allure embeds) applies an override only to a resource recorded before the snapshot (snapshotRenderer.ts resourceByUrl: the loop stops at a resource with _monotonicTime >= snapshot.timestamp, and the override is applied only if (result && GET)). So each image type gets one synthetic resource-snapshot at _monotonicTime: -1.
• The .invalid host never resolves (RFC 6761), so a viewer without override support fetches nothing.
• Why not core: this is trace-format writing, which lives in trace; no adapter changes.

Testing

• pnpm build, pnpm test (3290 passed) and pnpm lint are green; no workspace import leaks in the adapter or backend bundles.
• New tests: no data: or base64 in frame-snapshot events; every override and resource response names a file that exists in a real exported zip; our reader round-trips commands, filmstrip and a11y text unchanged and lists no synthetic request in the Network tab.

Not verified yet

• Opening a new zip in Playwright's viewer (trace.playwright.dev and npx playwright show-trace) and in Allure 2.35+, stepping through actions to confirm each shows its own screenshot. This needs a browser.
• Playwright's Network tab will list the synthetic action.png request once per image type. Our Network tab does not.

Screenshots / recordings

Not applicable: no UI change in our player. Screenshots from Playwright's viewer and Allure belong here once the check above is done.

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