Signed-Zoomify redirect coverage and redirect-vocabulary cleanup

[lovasoa]

Signed-Zoomify sites (Museum Ludwig / Kulturelles Erbe Koeln) answer the Zoomify metadata and every tile request with a 307 to a per-request signed CDN URL. Core support already landed (14e7a98, 398d1ff); this closes the remaining cleanup and coverage gaps.

refactor(protocol): drop the never-produced redirect-unavailable reason

Extension fetches follow redirects in the browser, so no layer reports this policy reason anymore. Removes the dead BlockedReason variant, its failure.ts/fetch.ts mapping branches, and regenerates packages/wasm-bindings/src/generated.d.ts via cargo xtask protocol generate. TRANSPORT_BAD_REDIRECT stays: the native transport still reports it for malformed redirect targets.

test(extension): reproduce the signed-proxy redirect shape end to end

The extension/tile-redirect scenario now mirrors the real flow: the Zoomify metadata and every tile 307 to a signed URL (previously only tiles redirected, behind a plain DZI). This pins "redirected metadata keeps the requested tile base" through a real browser fetch stack (packaged extension, headless Chromium, pixel-exact assembly) instead of only core unit tests. pyramid.dzi is replaced by ImageProperties.xml, tile bytes are unchanged, and the acceptance matrix maps the behavior to its lanes.

Scope notes

• No mock-level assertions of fetch options; a test that restates credentials/redirect values is a change detector, not behavior coverage.
• The website shares the same fetch semantics (credential omission, browser-native redirect following), and its metadata proxy intentionally refuses signed targets, so CORS-less sites stay extension-only by design.
• No live target for the museum: live checks are opt-in and advisory, and the deterministic corpus now models the exact shape.

Validation

• cargo xtask ci local and cargo xtask test all green (headless 6/6 including the retargeted signed case)
• cargo xtask check, cargo xtask fixtures verify (439 files / 58 scenarios)
• cargo xtask test core, cargo xtask test protocol, cargo xtask test extension