[MM-09] Add deterministic bounded MemorySegmentBuilder

[bug-superman]

MM-09: Add deterministic bounded MemorySegmentBuilder

Parent

#771

Checkpoint

MC-2 Bounded recall

What to build

Replace full active-memory concatenation with deterministic query, scope, freshness, conflict, dedupe, top-K, per-entry, and total-token selection.

Acceptance criteria

• Selection order matches the documented policy.
• Tie-breaking is deterministic.
• Superseded, contradicted, deleted, and out-of-scope entries cannot be selected.
• Per-entry and total budgets are hard limits.
• A selection failure falls back to a bounded safe result.

Required tests

• Score/tie property tests
• budget edges
• Unicode/empty query
• conflict and dedupe
• privacy negative cases

Benchmark

40-case recall subset, three paired reps; F1 >= 0.90, privacy 100%, tokens no higher than Current Maka.

Blocked by

• [MM-05] Refresh provider memory projection after committed changes #784 (MM-05)
• [MM-08] Converge durable-memory and Markdown compatibility contracts #787 (MM-08)

Rollout and rollback

Feature flag returns to the corrected legacy renderer; never return to cross-session or unbounded reads.

[Astro-Han]

Superseded by #1615 (automatic long-term memory lifecycle, SQLite-based; accepted by maintainers 2026-07-30, PR ① #1770 merged). This MM-09 work item never landed outside draft PR #820 and is replaced by the #1615 design and its LongMemEval/MSC evaluation plan.