[#196] Define storage TTL policy and safe terminal cleanup

[Baskarayelu]

Summary

• Define explicit TTL classes for instance state, active transfers, terminal transfers, caller allowlists, and account operation counters.
• Centralize persistent TTL bumps behind monotonic, threshold-based storage helpers.
• Move AccountOpCount to its declared persistent storage tier and apply a dedicated quota TTL.
• Add permissionless, idempotent terminal-transfer cleanup with a hard 20-id batch bound.
• Preserve pending escrow records and all settlement/accounting behavior; add policy documentation and regression coverage.

Design notes

extend_ttl is threshold-based and monotonic, so repeated writes are idempotent for healthy entries while records in their renewal window are restored to the class horizon. Terminal records use a shorter retention class for future terminal writes, while existing longer TTLs are never shortened.

Cleanup is direct-key and bounded. It only removes Claimed or Cancelled records, ignores pending/missing/repeated ids, never scans the transfer counter, and does not change escrow liability or account quotas.

Validation

• cargo test --lib — 140 passed, 4 pre-existing ignored.
• cargo fmt --all -- --check
• git diff --cached --check
• Budget regression measures a full 20-id cleanup request and enforces the default CPU ceiling.

Closes #196