feat(payments): add CEP-8 canonical invocation identity

[harsh04044]

Part of #100

Second piece of CEP-8, after the payment primitives in #102. This adds the canonical invocation identity: the hash that lets a paid authorization match a later retry of the same call.

The hash is SHA-256 over the RFC 8785 (JCS) canonicalization of { method, params }, with the top-level params._meta stripped. _meta is per-request MCP metadata (progressToken, stream) that the client regenerates on every call, so it must not affect identity. Same algorithm as the ts-sdk's computeCanonicalInvocationHash.

Still pure: no store, no middleware, no transport wiring. Just the two functions, the identity struct, and the error variant.

What's here

• compute_canonical_invocation_hash / _identity and CanonicalInvocationIdentity in src/payments/canonical.rs
• PaymentError::Canonicalize
• tools/gen-canonical-vectors/, a bun script that generates the golden fixture, plus the committed tests/fixtures/canonical_vectors.json (34 vectors)

Note:

• Why pin json-canon to exactly =0.1.3? Its output bytes gate cross-SDK grant matching, so a silent minor bump could break interop. It also has an i64::MIN bug (its bounds check does .abs(), which overflows), so canonical.rs runs its own safe-range pre-scan first and rejects any integer with |n| > 2^53-1 as PaymentError::Canonicalize before it reaches the engine. Hostile input fails closed instead of computing a hash a JS peer won't agree with.
• Where's the fixture from? The generator imports the ts-sdk's real computeCanonicalInvocationHash (not a copy, so it can't drift) against pinned canonicalize@2.1.0 + @noble/hashes@2.2.0. A golden test asserts our Rust output matches every vector byte-for-byte. It runs under bun because it imports a .ts file; the generator is offline only, and the crate and CI just read the committed JSON.
• No FFI change? The existing Error::Payment(_) wildcard arm already absorbs the new error variant, so the E0004 gate stays green and the C tests are untouched.

Heads up
Adding json-canon turns on serde_json's float_roundtrip feature crate-wide (Cargo feature unification). It's strictly more precise and is what float parity needs, it doesn't change integer or string parsing.