Add saved-tab question console mock

[isomorphisms]

Summary

• add a console question prompt over plain-text pages deliberately saved in the reading corpus
• build one readable exact vector index with canonical fixed-width text and a rebuildable Float32 query cache
• provide a deterministic 32-coordinate bounded-count model test double plus hooks for later embedding/LLM adapters
• run a one-member ensemble/reducer stage that records three checks and an aggregate score, followed by an explicit identity renderer
• store each exchange under questions and answers about tabs that the user has visited, separately from reading
• record per-query CPU/GPU and storage-path provenance; the current exact C scan reports dot-product-used-gpu=False

Review boundary

This is intentionally stacked on #26 (readable-vector-views). It reuses that readable multi-model vector backend instead of publishing the superseded binary-only backend from the earlier local branch. The bundled model is explicitly a deterministic test double, not an LLM, and the reducer is explicitly not XGBoost; both process boundaries are replaceable.

The mock reports one vector index and two equivalent query paths: authoritative readable text and the rebuildable Float32 cache.

Verification

• make clean test in native/vector-index
• sh bin/ci_browser_foundation.grease exercise-tab-qa
• sh bin/ci_browser_foundation.grease exercise-filesystem-views
• AddressSanitizer + UBSan vector smoke suite (detect_leaks=0; LeakSanitizer is unavailable in the ptraced workspace)
• exact local/remote tree SHA match before publishing