ui/synthetic-report: PLAN parity rule + testing doc

Locks in the synthetic-report parity rule as a global "Assumptions
and Defaults" entry in ui/PLAN.md: every phase that extends the
server->UI report contract must also extend the legacy parser in
the same PR (or document in tools/local-dev/legacy-report/README.md
why the new field cannot be derived from legacy text). The Go side
already enforces shape compatibility via the pkg/model/report
import; this rule extends that mechanical guard to "did we remember
to wire the new field through".

ui/docs/testing.md grows a "Synthetic reports for visual testing"
section with the full conversion -> load -> compose loop and the
two operational gotchas (no network on synthetic ids, page reload
clears the in-memory map).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

ui/PLAN.md

@@ -99,6 +99,25 @@ The intended v1 architecture is:
   format-compatible with GitHub Actions). Linux runners cover Tier 1
   tests; a macOS runner is provisioned only when Tier 2 iOS smoke is
   needed.
+- **Synthetic-report parser parity is a global rule.** A DEV-only
+  loader on the lobby (`import.meta.env.DEV`) lets the developer feed
+  the UI a JSON file that mimics a server `Report`, so the map and
+  inspectors can be exercised against rich game states without playing
+  many turns end-to-end. The JSON is produced by the Go CLI in
+  `tools/local-dev/legacy-report/`, which converts legacy text
+  reports under `tools/local-dev/reports/` into the shape of
+  `pkg/model/report.Report` (whatever subset the UI currently
+  decodes). Every phase that **extends the server→UI report contract**
+  — adding decoding for a new `Report` field in
+  `ui/frontend/src/api/game-state.ts` — must, in the same PR, extend
+  the legacy parser to populate that field, **or** explicitly note in
+  the parser's `README.md` that the field cannot be derived from the
+  legacy text format and is left empty in synthetic JSON. The point
+  is to keep `tools/local-dev/legacy-report/` a faithful (and
+  type-checked, via `pkg/model/report` import) generator of test
+  inputs as the UI grows; otherwise synthetic data silently lags
+  behind the contract and visual tests stop covering the new
+  behaviour.
 
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