chore: seal Design Vision v1 and monorepo scaffold

Archive the differentiated YuXinGu product docs, AI engineering system,
design contract, and Go/Vue scaffold. Next execution prioritizes Cece-parity
over early innovation (see .ai/product/STRATEGY.md).

Co-authored-by: Cursor <cursoragent@cursor.com>

.ai/testing.md

@@ -0,0 +1,22 @@
+# Testing — Golden Rules
+
+## Priority
+
+Test money, scoring, auth, and unlock paths first.
+
+## Go
+
+- Pure engines (decode / scale scoring) must have unit tests.
+- `go test ./...` must pass before claiming API done.
+
+## H5
+
+- Critical utils may use Vitest when introduced.
+- Until then: PR must list manual smoke steps for touched flows.
+
+## Definition for AI
+
+Do not mark a feature Done if:
+
+- Scorable logic has zero tests, or
+- Build is broken (`go test` / `npm run build:h5` fail).