Deterministic architecture / impact map

[JSONbored]

Phase 4 — Self-host-native differentiators

Context

Reviews reason mainly about the diff; we have a RAG index (src/review/rag.ts, rag-index.ts) that already knows callers and related modules.

Goal

A deterministic "impact map" showing which modules/callers a PR touches, to push review beyond the diff.

Requirements

• From the RAG index + changed symbols, compute affected modules/callers deterministically; render a compact impact section in the unified comment; optionally feed grounding. An AI-generated diagram may be added later behind a separate default-OFF flag.
• Bounded, fail-safe, default-OFF (review.impact_map).

Deliverables

• Impact computation + renderer + grounding hook + tests + docs.

Expected outcome

• Reviewers see cross-file blast radius; catches integration risks the diff hides.

Effort

L.

[linear-code]

JSO-2051