[pr-devserver-review] Walkthrough 4/4 — ADR, CONTEXT.md terms and spec Revision 4

## Parent

FR-3313. Design: wayfinder map FR-3941 — every decision (FR-3942–FR-3946 resolution comments and the map's Notes) is the source. Documents only; no code.

## What to build

- **`docs/adr/0004-walkthrough-stops-in-the-v3-anchor.md`** (title per `adr-writing`): the decision that a Stop is an additive set of optional v3 anchor fields (no v4 envelope), strict resolution for stops, the codec-owned volatile-query denylist, the 20-stop cap and the comment-limit reasoning, the `` marker kept apart from `bai-review`, and the guided mode as the docs PR preview's grammar ported as design (shared implementation out of scope). Rejected alternatives and consequences from the tickets. Row in `docs/ARCHITECTURE.md`.
- **`react/vite-plugins/review-overlay/CONTEXT.md`**: widen **Pin** ("one remark tied to one element", whoever authored it); add **Walkthrough**, **Stop**, **Mark**, **Navigator**; keep the `_Avoid_` style.
- **`frontend-board/specs/pr-devserver-review.md`** (separate PR in `lablup/frontend-board`): a `# Revision 4 (2026-09-15)` section in the R3 shape — supersession statement (R3.1's "no ✅ What to check comment" is revisited as a walkthrough with a webui-owned trigger, no dw change, no notification; R3.9 stays dropped), numbered `R4.n` subsections per decision cluster, a work-item impact list naming FR-A/B/C, and extension notes (static build, My board card line).

## Acceptance criteria

- [ ] Every Decisions-so-far bullet on FR-3941 is represented in the ADR or R4 with its ticket linked.
- [ ] `CONTEXT.md` stays a glossary (no implementation details).
- [ ] The ADR passes the `adr-writing` landing checks; `docs/ARCHITECTURE.md` lists it.

JIRA Issue: FR-3952

Start with FR-3313 and FR-3941's FR-3942–FR-3946 resolution comments and map Notes, then review the ADR format and `adr-writing` checks. Update `docs/adr/0004-walkthrough-stops-in-the-v3-anchor.md`, `docs/ARCHITECTURE.md`, `react/vite-plugins/review-overlay/CONTEXT.md`, and the separate `frontend-board/specs/pr-devserver-review.md` revision. Done means every decision is represented with its ticket linked, CONTEXT remains a glossary, and the architecture index lists the ADR.

Written by the indexing model from the issue text.