feat(Schonhage): compose routed native box products with shared geometry #2314

Continue #1394 by connecting the existing native Wanted router and shared band producer to the new box-query batch (#2312). Owner/session codex-root-3sum-20261102, branch work/three-sum-native-box-product-20261102 from reviewed32eaaf0; published supplier branches stay unchanged.

Acceptance: one fixed standard WordRAM program takes original signed matrices and coordinate occurrences plus an ordinary threshold, computes actual wanted codes/spans and shared band encodings, constructs box geometry once, computes retained box values once per tile, executes actual sparse queries and reconstructs signed matrix-product answers in original request order. Charge geometry once, per-tile table construction, every query occurrence and all real argument/copy/restore/loop instructions. Derive fits/width from original scalar and magnitude bounds, preserve the original input and outside frame, handle empty matrices/requests and duplicates, and test initialized signed/dirty-memory behavior. Reuse Wanted/Product/Band/BoxTable/BoxBatch contracts; no supplied solver, table, layout, execution, time or intermediate-fit oracle. Independent correctness/API/resource review and full six-gate validation are required before integration.

This completes a native all-tile composition milestone, not the full quantitative endpoint. The improved Corollary26/thin-product estimate, ET replacement, parameter retuning and ordinary-3SUM exponent1.9992 remain #1394 unless separately proved and explicitly reported. No curated abstract change expected. AI assistance: OpenAI Codex with disjoint implementation subtasks and independent review.