Minimal protected-endpoint verifier SDK for Rust, aligned to the Python
kernel's public action_intent and pccb contracts.

Mirrors the Rust SDK from actenon-kernel/sdk/rust/ into its own
repository so Rust users can use it as a git dependency:
  cargo add --git https://github.com/Actenon/sdk-rust

crates.io publication is prepared (Cargo.toml has all required fields;
publish workflow is included) but pending the CARGO_REGISTRY_TOKEN
secret. Once the token is added, pushing a v* tag will publish
automatically.

Scope:
- action_intent v1 and pccb v1 Rust data structures
- protected-endpoint proof verification
- exact audience, tenant, subject, action, target, action-hash,
  not-before, and expiry checks
- optional verifier-side clock skew tolerance (default zero)
- deterministic local-proof verification (HS256)
- custom signature verification via SignatureVerifier trait
- offline Receipt counter-signature verification
- offline, fail-closed issuer-status verification
- signed exact-action approval verification