Carry ProvenanceAttestation on the wire (frames envelope + JSON Schema)

[macanderson]

PR #87 defines ProvenanceAttestation and the constructions behind it, but no envelope transports it — a provider that signs a frame has nowhere to put the signature, so the feature is currently reachable only by out-of-band agreement.

Needed:

• A place in the frames envelope for per-frame attestations and/or one result-set Merkle root attestation. Per F6 it stays detached: beside the frame, never inside it, so re-signing and key rotation never perturb the content-addressed identity.
• The matching schema/contextgraph-envelope.schema.json definitions.
• A wire example under examples/, which GOVERNANCE.md requires as the witness for a normative change.
• Decide whether a signed result set carries inclusion proofs inline or a host derives them from the frames it received.

Additive within contextgraph/1 (new optional members), so no family bump.

Definition of done

• ContextQueryResult carries the attestation, so it travels on every binding (stdio, HTTP, in-process) rather than only where an Envelope is built.
• Per-frame evidence is a list of entries that each name the (provider_id, frame_id, content_digest) identity in full, never a parallel array indexed by position.
• A single result-set attestation over the RFC 6962 Merkle root is representable.
• Nothing added is required: an unsigned result serializes byte-for-byte as it did before, and a consumer that ignores the new members still parses a signed envelope. Covered by a test for each direction.
• The attestation never appears inside a ContextFrame (F6) — asserted on the serialized bytes, not on the struct layout.
• schema/contextgraph-envelope.schema.json defines the new members and their types, and python3 schema/validate-examples.py exits 0.
• A signed wire example ships in both examples/reference-messages.json and examples/full-stdio-session.ndjson, and the reference-serialized vectors gain an attested line.
• Every signature and Merkle root shipped in this repository is recomputed and cryptographically verified by a test, against a published example key — not asserted.
• A witness test fails without the change and passes with it, checked by reverting the implementation.
• The inclusion-proof decision (inline vs host-derived) is made and its reasoning recorded in an ADR under docs/adr/, with the ADR listed in the docs/GUIDE.md decision log.
• SPEC.md states the wire carriage normatively, with new requirement anchors appended (never renumbering F1–F10).
• CHANGELOG.md records the change under ## [Unreleased].
• docs/protocol-surface.md's ContextQueryResult snippet matches the Rust type.
• No protocol family bump: the change is additive within contextgraph/1.
• cargo fmt -- --check and cargo clippy -p contextgraph-types -p contextgraph-conformance --all-targets are clean, and the touched crates' tests pass.

[macanderson]

Note from the host side (#88 / #91, ADR 0016): the consumer for this field now exists, so when the envelope lands there is a definite place to put what it parses.

ContextProvider::query_attested (contextgraph-host/src/provider.rs) returns an AttestedQueryResult { result, attestations: Vec<FrameAttestation> }, where FrameAttestation binds one ProvenanceAttestation to a ContextFrame::id. It is a defaulted trait method returning no attestations, which is the honest answer for every provider in the workspace today — including StdioProvider and HttpProvider, which parse none because the envelope carries none.

So this issue has a host-side half that is easy to miss: StdioProvider and HttpProvider must override query_attested to hand over what they parsed, or attestations will land on the wire and still reach no verifier. Both live in contextgraph-host/src/{stdio,http}.rs. The rest of the path — trust-store lookup, per-frame verification, the CompositionAudit field — is already wired and needs no change.

FrameAttestation is defined in contextgraph-host/src/trust.rs today with a comment saying it belongs in contextgraph-types once this lands; moving it there is the natural shape.

Worth a test in the same PR: a stdio provider fixture that serves a signed frame, and a host holding its key, composing to an audit entry that reads Attested. That closes the loop end to end rather than at the parser.