test(fixtures): a shrunken fixture passed green in all four ports

[janakhpon]

• replace every emptiness check on a shared fixture with a floor on its count
• cover merge 18, mutations 21, rule 23, tiling 14, probes 3, dilate 12, kernels 5
• state why a floor and not equality, in each language
• re-run every suite, and mutation-test each port against its own fixture

Every consumer asserted the fixture was non-empty and none asserted how much of it was there. A fixture rebuilt holding three of its eighteen cases is not empty, so web, iOS, Android and Rust would all have gone on passing while testing a sixth of what they advertise. That is the defect the fixture header warns about, running in the checks meant to prevent it.

Floors rather than equality, and the reason is in each file. A fixture GAINING a case is the normal and wanted direction, and pinning equality would make every addition an edit in four languages. What a floor cannot catch is a swap, one case removed and another added, and equality would not catch that either.

Mutation-tested per port on exit code, each against its own fixture and each restored byte-identically afterwards. Rust 18 to 3 panics with the count in the message, web 23 to 2 exits 1, Android 14 to 2 exits 1, and iOS 18 to 4 exits 1. The iOS arm was worth running separately because a Swift mutation earlier in this work trapped rather than failing, and a harness reading reported failures scored it as a survivor.

Green afterwards across all three suites, at 107 web tests, 73 iOS tests in twelve suites, and 106 Android tests.

What changed

Why

How it was verified

• pnpm --filter ./apps/web test
• pnpm --filter ./apps/web run lint
• cd services/feedback && go vet ./... && go test ./... (if Go changed)
• Android unit tests, if Android changed: cd apps/android && ./gradlew testDebugUnitTest
  (needs JAVA_HOME on Android Studio's JBR; see apps/android/README.md).
  CI does not run them, so this is the only gate on that suite.
• iOS MonOcrCore tests, if iOS changed: cd apps/ios && sh Scripts/swift-test.sh.
  The ios-core job runs these on every push, so this is a faster copy of a
  gate that does exist. The app target is still not built anywhere.

Claims