[FEATURE] Verify ML-DSA signatures embedded in provider docs (x-stackql-signature), with .sig fallback #153

Middle step of the registry's move to quantum-safe, embedded provider-doc signatures: stackql/stackql-provider-registry#482 (signing package) -> this issue (verification) -> stackql/stackql#856 (bump, config, docs). Verification is off by default in stackql (nopVerify: true), so this ships without changing user-facing behaviour. omnisdk, which is replacing any-sdk, gets the same verifier under stackql-labs/omnisdk#17.

NOTE: Must be compliant with at least FIPS 204.

What changes

• internal/anysdk/registry.go: getVerifiedDocBytes fetches <doc>.sig (local path, local cache or remote) and calls edcrypto.VerifyFileFromCertificateBytes. New path, tried first: if the doc carries x-stackql-signature, pop it, canonicalise the remainder with the registry's shared package (RFC 8785 JSON), mldsa.Verify against the embedded public key selected by kid, context stackql-provider-doc-v1. No key: fall back to the .sig path. Both present: embedded wins.
• Dependency: bump github.com/stackql/stackql-provider-registry from v0.0.2-alpha01 to the release that ships signing/pqsig. go.mod to go 1.27 for crypto/mldsa (currently 1.26.10).
• VerifierConfig: add allowLegacySig (default true for the transition) so the .sig fallback can be switched off later. nopVerify untouched.
• Canonicalisation comes from the shared registry package. Do not reimplement it here or signatures will not match.
• Tests: fixtures signed at test time with ephemeral ML-DSA keys. Cases: valid, tampered after signing, unknown kid, no key with and without .sig, both present.

Sequence and dependencies

1. Blocked by [FEATURE] Quantum-safe provider doc signing: ML-DSA via crypto/mldsa, signature embedded in the doc, drop .sig sidecars stackql-provider-registry#482 step 1 (package release).
2. This issue, then an any-sdk release.
3. Unblocks [FEATURE] Verify ML-DSA signatures embedded in provider docs (x-stackql-signature), replacing .sig sidecars stackql#856, which in turn unblocks the registry dropping .sig files.

Notes

• Verification stays at doc-read time. Archive pull stays unverified; that is [SECURITY] Define and enforce registry content verification before installation and execution stackql#832's scope.
• Provider docs drive execution (local_templated and friends), so coverage must stay per doc, not only provider.yaml.