feat: land the store-free admission composite

[mfw78]

What does this PR do?

Rewrite the stamp validator against the callers that exist: a store-free admission composite, the canonical gate a node's reserve runs against an already-loaded batch, shipped where the security core belongs in the proving lane.

Changes

• Add AdmissionValidator in crates/postage-primitives/src/admission.rs: the admission composite, store-free by design, gating a stamped chunk against a loaded batch in a fixed order: the batch the stamp claims, the usability gate, the expiry gate, the index bounds, the bucket match, the owner signature.
• Add AdmissionError to the crate error home: one variant per denial, #[non_exhaustive], the Stamp wrap carrying its #[from] source, and is_context_dependent as the retryability predicate.
• Guard the classification with an exhaustive classification test and a source-chain test in the crate's house pattern.
• Re-export both names at the nectar-postage facade.
• Cover the composite with generated-stamp tests: accept, the identity gate, the threshold, the expiry, a foreign owner, the geometry wraps and the signature wrap.

Breaking changes

None. Additive. The trait this replaces, StampValidator, was deleted in M1 with zero implementors.

Testing

• Unit tests pass: 333 green across the four postage crates, including the nine new admission and error tests
• cargo clippy --all-targets -- -D warnings clean, including the nectar-postage-primitives serde, parallel and arbitrary feature paths; the reinvention gate, fmt and the doc build under -D rustdoc::broken_intra_doc_links clean
• Manual testing completed: the surface is type-level; the tests are the check
• Documentation updated

Related issues

• The M1 inputs document (docs/restructure/m1-trait-redesign-inputs.md) holds the locked decisions this lands: the leaf predicate (D2), the store-level context gates (D3) and the one canonical composite (D4), with the node's standalone admission implementation as the design input.
• The nectar-postage-api car (api: create the tier-two trait crates with noop implementations #812) moves this surface into the fresh crate.

AI assistance disclosure

AI assistance: Claude Code (opencode) used for the design research, the implementation and this body.

Notes for reviewers

• The gate order is fixed and leading: the batch-identity check first, a gate the node's design input lacked, because a foreign-batch stamp signed by the same owner would otherwise clear the signature check.
• UnknownBatch is the caller's own load-miss variant; the store-free composite never yields it, and no store-coupled path ships in this car.
• The test stamps are signed at run time by nectar's own signing path over the deterministic EIP-191 key bee's signer tests pin; the module doc states the fixed inputs and why no upstream vector exists for the composite's answers.

Checklist

• Code follows project style
• Self-review completed
• Tests added/updated
• No console.logs or debug code left behind
• PR title is descriptive

Closes

• Closes postage-api: rewrite the stamp validator against its real caller #835