docs+ci(predictive-echo): CI package-suite step, invariants, changeset

ci.yml runs the xterm-zerolag-input suite (Layers 1-3) after the root
npm ci (workspaces hoisting; no separate install). CLAUDE.md and
architecture-invariants.md rewrite the codex echo story: predictive
write-through with the wire-neutrality, separate-bundle, composer-gate,
baseY and blank-neutral invariants spelled out; the single-source section
now covers both vendor bundles and why their entry points differ.
Changeset: minor for aicodeman + xterm-zerolag-input.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -91,6 +91,14 @@ jobs:
         # Safe in CI: TmuxManager no-ops all shell commands under VITEST (test/setup.ts).
         run: npm run test:ci
 
+      - name: Run xterm-zerolag-input package tests
+        # Layers 1-3 of the predictive-echo suites (unit laws, fixture replay,
+        # seeded fuzz): deterministic, no browser, no live server. Depends on
+        # the ROOT `npm ci` above — workspaces hoist the package's vitest into
+        # the root node_modules; do not add a separate install here.
+        run: npx vitest run
+        working-directory: packages/xterm-zerolag-input
+
 # Note: The browser-driven mobile suite (test/mobile/**) is excluded from CI —
 # it needs a live server + chromium + environment-specific PNG baselines.
 # Run it locally/manually. All other tests run via the `test` job above.