stack-encrypt conformance: specify and test context composition #1475

The initial read profile covers context preflight, but does not establish the complete composition contract needed to refactor records safely.

Acceptance:

• Add shared Rust/Go cases for literal and field-derived bases, caller input, nested/composite context, and caller extensions; fix ordering and grouping explicitly.
• Distinguish a stored-context expectation from an extension. Cover required/missing inputs, mismatch, stored-context tampering, queries, and empty/late-failure batches.
• Compare canonical descriptors/AAD and observable results; assert rejection before key retrieval where validation is independent of keys.
• Exercise typed/generated APIs as well as dynamic lowering and require every enrolled case in CI.
• Document current semantics separately from future ciphertext-binding/search-scope roles.

Next implementation slice after #1474. Reuse #1473 and existing context tests; reconcile the repeated-extension report #1120 rather than freezing a known divergence as desired behavior.

Plan: conformance roadmap and API redesign #1365.