P0.4: wire the logic verification gate (sx assert helper + snapshot runner)

Add a real logic-test gate so future pure-sx game logic fails the build on a
bad assertion:

- tests/test.sx: `expect(cond, msg)` assert helper — prints a greppable
  `FAIL <file>:<line>: <msg>` and exits non-zero via process.exit on failure.
- tools/run_tests.sh: snapshot runner mirroring sx/tests/run_examples.sh; runs
  each tests/<name>.sx and diffs stdout + exit code against tests/expected/.
  Exits 0 iff all tests pass.
- tests/arith.sx (+ expected snapshots): seed passing sanity test.
- README.md: document both halves of the gate — logic runner and the
  reproducible ios-sim build/launch sequence (with device discovery).

README.md

@@ -7,3 +7,53 @@ A match-3 game written entirely in the **sx** language, targeting **iOS** first.
 - Verification gate: sx logic tests pass AND the iOS app builds & launches in the Simulator.
 
 Development is driven by the multi-agent `flow` (Product Owner → Worker → Reviewer → Observer).
+
+## Verification gate
+
+The gate has two halves. Both must pass. The sx compiler used below lives at
+`/Users/agra/projects/sx/zig-out/bin/sx` (override the runner's binary with the
+`SX` env var). Run everything from the repo root.
+
+### 1. Logic tests
+
+Pure-sx logic tests run under `sx` and have their stdout + exit code diffed
+against committed snapshots in `tests/expected/`. A failed assertion exits the
+process non-zero, so it fails the runner (and the gate).
+
+```bash
+bash tools/run_tests.sh
+```
+
+- A test is any `tests/<name>.sx` that has a `tests/expected/<name>.exit`
+  marker; `tests/test.sx` (the `expect` assert helper) has no marker, so it is
+  not itself run.
+- Regenerate snapshots after an intentional change: `bash tools/run_tests.sh --update`.
+
+### 2. iOS Simulator build + launch
+
+Build the app for the simulator, then install/launch it on an available device
+and screenshot the rendered scene (blue background + a centered orange quad).
+
+```bash
+# Build the .app bundle (sx-out/ios/M3te.app):
+/Users/agra/projects/sx/zig-out/bin/sx build --target ios-sim main.sx
+
+# Discover an available simulator — do NOT hardcode a udid:
+xcrun simctl list devices available
+# e.g. capture the first available device's UDID into $udid:
+udid=$(xcrun simctl list devices available | grep -Eo '[0-9A-Fa-f-]{36}' | head -1)
+
+# Boot it (skip if already "Booted") and bring the Simulator window up:
+xcrun simctl boot "$udid" || true
+open -a Simulator
+
+# Install, launch (bundle id co.swipelab.m3te), and screenshot:
+xcrun simctl install booted sx-out/ios/M3te.app
+xcrun simctl launch booted co.swipelab.m3te
+xcrun simctl io booted screenshot /tmp/m3te.png
+```
+
+The screenshot should match `goldens/p0_quad.png` (a centered orange quad over a
+blue clear), modulo the status-bar clock — pixel-exact equality is not required.
+A tap on the quad flips its color (orange ↔ green); see
+`goldens/p0_input_before.png` / `goldens/p0_input_after.png`.