feat(review): compute deterministic impact map from RAG index + changed symbols

[JSONbored]

Compute slice of #1971: given changed symbols and the RAG index, deterministically resolve the affected modules/callers (blast radius). Pure computation over the existing vector/BM25 retrieval — no rendering, no grounding wire yet. Bounded (cap affected modules), fail-safe, default-OFF-guarded by the caller.

Deliverables

• New src/review/impact-map.ts exporting computeImpactMap(symbols, ragContext) -> {changedModule, affectedModules[], callers[]}[] using retrieveContext/bm25Rerank (src/review/rag.ts:470, src/review/rag.ts:511)
• Deterministic ordering + a hard cap on affected modules per changed module
• Fail-safe: RAG unavailable or empty index => empty impact map (no throw)
• Tests: single-caller, multi-caller, self-only, empty-index, cap enforcement (branch-counted)

References

• src/review/rag.ts:470
• src/review/rag.ts:511
• src/review/rag-wire.ts:173

Part of #1971.

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size: S · held for later release (gittensor:feature once activated)

[linear-code]

JSO-2284