feat(memory): add decoder-backed recall budgeting and density gates

[tcconnally]

Summary

• Add decoder-backed selective recall budgeting that preserves load-bearing memories and content-free decoder references.
• Add deterministic offline semantic-density gates plus sanitized real-Vault persistence replay.
• Add bounded, measurement-only real-Vault retrieval probes with privacy checks and signed reports.

Verification

• Focused regression and benchmark tests pass locally.
• Semantic-density gate: PASS.
• Real-Vault persistence density gate: PASS.
• Probe replay repeated twice on fresh databases with identical rows and measurement signature.
• No Greg/AWS/network calls used.

Scope

• Generated perseus.py rebuilt from source.
• Separate perseus-vault worktree was not modified.
• Sanitized corpus contains no operational metadata or detected credential/contact/path patterns.