feat(hosted): add retention and measured scale gates

[AetherAI3]

The hosted context foundation lacked enforceable retention and honest production-scale evidence. A stale hold release could race newer policy, checkpoint deletion and key destruction were not independently bound, build and benchmark freshness could drift, and local predicate checks could be mistaken for hosted data-plane proof.

This change adds revision-CAS retention commands, per-cycle versioned DEKs, remote-object deletion verification before independently receipted managed-KMS destruction, and crash-safe cleanup owned by the original operation identity. Managed-key rotation preserves the immutable key version for every opaque key reference and requires a current signed provider qualification for every version still used by a non-expired cycle. It also binds checkpoint/hydrate identity, runtime package-tree bytes, the exact data-plane case generator, and live hosted-readiness evidence.

The clean-source measurement harness records capacity, recall, retrieval latency, resident memory, checkpoint, hydrate, and actual index rebuild. The deterministic one-million-case local isolation run is explicitly predicate evidence and cannot qualify reach; qualification requires a separate independently signed hosted data-plane receipt. FileCycleKeyProvider remains canary-only. wheel_digest is signed provenance; the runtime enforces the imported Python package-tree digest, while CI or release custody must independently verify a claimed wheel artifact.

Validation:

• python -m pytest -q — 637 passed, 5 skipped
• python -m pytest -q tests/test_retention_scale.py — 24 passed
• python -m pytest -q tests/test_hosted_context.py — 13 passed
• Focused cleanup crash/replay and overlapping managed-key rotation regressions — 3 passed
• Ruff, mypy for aether_context, compileall, schema regeneration, and git diff --check — passed

Live provider credentials, managed infrastructure, a randomized million-case hosted data-plane receipt, and the Gate 8 observation window remain external rollout gates. This PR does not claim those gates have passed.