feat(memory): add explicit corpus inspection

[BrendanChou]

Summary

• add a fixed-size, telemetry-neutral inspect operation for explicit memory-corpus audits
• return compact attributed records with exact keys and best-effort cursor pagination
• keep full content behind targeted reads and keep inspection separate from the scan-before-put gate
• teach built-in Reflection to enumerate memory only when the user explicitly requests an audit, cleanup, or inspection
• report backend scope, coverage, truncation, and uncertainty without mutating memory

Remote inspection preserves own-versus-other attribution from the selected backend and advances safely across pages emptied by secret suppression. Ordinary reflection and relevance-gated scan/read behavior remain unchanged.

Testing

• just check-fmt
• cargo check --all-features
• cargo test -p tact-memory (89 tests)
• cargo test -p tact (762 unit tests, 15 release-pipeline tests)

Stack

Depends on #156. Retarget to main after #156 merges.