Gecko WebCrypto admission laxness: cross-curve EC PKCS#8, OKP crv, HMAC kty — host guards + upstream reports

[lann]

Running the conformance suites under Playwright Firefox (the jco-firefox target) surfaced three classes where Gecko's WebCrypto admits key material the spec's import steps reject (and every other engine rejects). The suite pins rejection (invalid-key), so these appeared as must-reject violations on Firefox:

1. Cross-curve EC PKCS#8: importKey("pkcs8", <P-256 key>, { name: "ECDH"|"ECDSA", namedCurve: "P-384" }) succeeds (probe/ecdh-format-roundtrips, probe/ecdsa-private-format-imports).
2. OKP JWK crv confusion: an X25519 JWK imports as Ed25519 and vice versa (probe/sig-public-format-imports, probe/x25519-key-contract).
3. HMAC JWK kty: a JWK with the wrong kty mints an HMAC key (probe/jwk-rejections).

Disposition: per the portability contract's resolution ladder (strategy 2 — enhance the deficient implementation with logic, never crypto), the jco host now carries shallow fail-closed admission guards for all three, in the mold of its existing requireNamedCurveSpki/requireRsaEncryptionSpki checks: a fixed-shape PKCS#8 AlgorithmIdentifier compare, and kty/crv member checks on the parsed JWK. They can only over-reject; whatever passes still gets the platform's full validation. The conformance probes above are the vector coverage, gating on every engine leg.

Remaining work this issue tracks:

• Report the three Gecko behaviors upstream (Bugzilla), link the reports here.
• When Gecko fixes land in the Playwright pin, consider whether the guards stay (they are cheap and uniform; likely yes).

[lann]

Note for the deltic host module (js/deltic): the admission guards this round added to the jco host (cross-curve EC PKCS#8, OKP JWK crv/kty, HMAC JWK kty — PR #359) live in js/jco/webcrypto.js only. The deltic targets currently run on Chromium and Deno, whose platforms reject these imports natively, so no gap is observable today — but a deltic target on Gecko would need the same guards mirrored into js/deltic/src/mod.ts. The conformance probes that pin the behavior (probe/ecdh-format-roundtrips, probe/ecdsa-private-format-imports, probe/sig-public-format-imports, probe/x25519-key-contract, probe/jwk-rejections) would catch it at target-addition time.