feat(memory): rank private recall with bounded hybrid signals and safe fallback

[robinbraemer]

Outcome

Combine approved lexical, optional semantic, metadata, and bounded recency signals into an explainable private-memory ranker with safe fallback.

Scope

• Define a versioned deterministic scoring contract and fusion method.
• Preserve pinned-topic behavior and the existing attachment count/byte ceilings.
• Evaluate current LLM selection as fallback, reranker, or removal candidate.
• Deduplicate overlapping chunks and attach only validated source topics.
• Record bounded method, counts, timing, and degradation telemetry without content.

Acceptance criteria

• Identical index/configuration inputs produce stable ranking.
• Each selected result can report safe score components and source path to the owning Mate.
• Hybrid retrieval meets the benchmark's quality and resource gates.
• Failure of any optional signal produces a defined lower-capability path.
• No recalled chunk is attached after its source changed without revalidation.