docs(testing): dedupe Section 9 multi-context line; minor decision-tree clarity

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

dev-docs/ARCH-TESTING.md

@@ -79,6 +79,8 @@ is the most expensive tier.
 
 ## 2. When to use what — decision tree
 
+*First match wins — stop at the first YES.*
+
 ```
 Is the thing under test pure logic with no DOM?
   └─ YES → Unit (Vitest + happy-dom)
@@ -136,7 +138,7 @@ stabilization B5).
 pnpm test:visual:update
 ```
 
-Reviews PNG diffs in PRs. Baselines live at:
+Diffs are reviewed in PRs. Baselines live at:
 ```
 apps/app/tests/playwright-ct/__screenshots__/visual/<spec-path>/<name>.png
 ```
@@ -333,8 +335,7 @@ For Playwright tests to run, the host must have:
 - F4 isolated component-level visual baselines (replacing the
   composite baselines in B3 with per-state baselines using stable
   `data-test-id` attributes).
-- F4 multi-context concurrent-edit e2e tests (currently the 409
-  contract test uses single-context replay).
+- Multi-context concurrent-edit e2e patterns — see TEST-INFRA-002 in BACKLOG.md
 - Multi-browser (Firefox, WebKit) baselines — Linux+Chromium only
   for v1 per RFC §A.5.
 - Mobile viewport baselines — desktop 1440×900 only for v1.