Port provenance attestation to the TypeScript, Python, and Go SDKs

[macanderson]

SPEC.md §6.5.1 defines a normative byte encoding, and contextgraph-types/tests/attestation_vectors.rs publishes the vectors to reconcile against. Right now Rust is the only implementation, so the cross-language claim is untested — and an encoding rule with exactly one implementation is indistinguishable from an implementation detail.

Per SDK: the length-prefixed link encoding, the source-first chain fold, the frame commitment, the RFC 6962 Merkle root and inclusion proofs, and Ed25519 verification. Each must reproduce the published vectors byte-for-byte in its own test suite.

The encoding was chosen over RFC 8785 (JCS) precisely so this port needs no JSON canonicalizer — if any SDK finds itself reaching for one, the encoding has been misread.

Highest value on the two dynamic-language SDKs, where a subtle divergence (UTF-16 length instead of UTF-8 byte length, signed 32-bit prefixes) is easiest to introduce and hardest to notice.

Definition of done

Each box is checkable before the merge, by running the command it names.

The vectors can fail a wrong port. The published set was ASCII-only, its
only multi-leaf Merkle vector had a power-of-two leaf count, and it carried no
signature and no inclusion proof — so a port could get all three of the named
traps wrong and still go green. Fixing that is part of this issue, not a
follow-up.

• The vector set gains a link whose fields are multi-byte UTF-8 ending in
  an astral-plane character, so the UTF-8-vs-UTF-16-vs-code-point length
  question has an oracle
• The vector set gains one-, three- and seven-leaf Merkle roots, so RFC
  6962's split is distinguishable from the duplicate-the-last-leaf shortcut
• The vector set gains an inclusion proof and a fixed Ed25519 key with the
  signature it produces, so §6.5.3 and §6.5.4 have oracles at all
• No previously published value changes (that would be a wire break,
  SPEC.md §15)
• The vectors live in one file every language suite reads, with the Rust
  reference asserting the file agrees with what it publishes inline

The ports. Each covers the length-prefixed link encoding, the source-first
chain fold, the frame commitment, the RFC 6962 root and inclusion proofs, and
Ed25519 verification.

• TypeScript — sdk/typescript/src/attest.ts, exported from the package
  index
• Python — contextgraph_sdk.attest, exported from the package index
• Go — sdk/go/contextgraph/attest
• No port reaches for a JSON canonicalizer (ADR 0010: the encoding was
  chosen so none is needed)
• No SDK gains a third-party dependency

Each port reproduces the vectors in its own suite.

• TypeScript: cd sdk/typescript && npx tsc && node --test "dist/test/*.test.js"
• Python: cd sdk/python && python3 -m unittest discover -s tests -v
• Go: cd sdk/go && go test ./contextgraph/attest/
• Rust: cargo test -p contextgraph-types --features attestation
• Every SDK asserts against the strings the shared fixture publishes, not
  against values it computed itself
• Each suite is demonstrated to fail on a wrong implementation, not only
  to pass on the right one — transcripts in the PR

Wired into CI.

• Each of the three SDK jobs in .github/workflows/ci.yml runs its
  attestation suite
• CI compiles and runs the Rust vectors, which the default-feature
  workspace run never did

Documented.

• Each SDK README shows verification and names that SDK's own trap
• CHANGELOG.md records the ports under [Unreleased]

[macanderson]

The wire shape to port has landed in #138 (ADR 0014, SPEC.md §6.5.5): ContextQueryResult gains frame_attestations (a list of { frame: FrameId, attestation?, inclusion_proof? }) and result_attestation. Both are optional and omitted when empty, so an SDK that ports nothing stays conformant — but an SDK provider still cannot sign what it serves until it can emit these two members. schema/contextgraph-envelope.schema.json carries the definitions, and examples/reference-messages.json (id: "q3") is a signed exchange whose signatures a port can reconcile against; the example public key is published in examples/README.md.