Wire KMP and MAUI FireOS/Horizon e2e build targets

[hyochan]

Context

The expanded e2e-tests workflow now requires every library/platform row to be reported explicitly. During that audit, the KMP and MAUI rows for FireOS/Amazon and Horizon had to be marked as blocked because there is no build switch or flavor wiring for those targets yet.

This is different from react-native-iap, expo-iap, and flutter_inapp_purchase, which already expose Android store selection through flags such as fireOsEnabled, horizonEnabled, EXPO_IAP_FIREOS, and EXPO_IAP_HORIZON.

Work to do

• Add FireOS/Amazon and Horizon build wiring for libraries/kmp-iap.
• Add FireOS/Amazon and Horizon binding/example wiring for libraries/maui-iap.
• Make the target selection explicit so the e2e workflow can build these rows instead of reporting them as blocked.
• Update .claude/commands/e2e-tests.md once real commands exist.
• Verify the normal Android/Play and iOS paths still build after adding the new target switches.

Current blocked rows

KMP FireOS   | BLOCKED | no FireOS/Amazon flavor switch wired
KMP Horizon  | BLOCKED | no Horizon flavor switch wired
MAUI FireOS  | BLOCKED | no Amazon binding/example flavor switch wired
MAUI Horizon | BLOCKED | no Horizon binding/example flavor switch wired

Acceptance criteria

• KMP has documented commands for Android/Play, FireOS/Amazon, Horizon, and iOS e2e build rows.
• MAUI has documented commands for Android/Play, FireOS/Amazon, Horizon, and iOS e2e build rows.
• e2e-tests can report these rows as PASS or FAIL based on real commands, not BLOCKED: not wired.

Follow-up note

Pick this up after PR #162 is merged. There is already in-progress work for these target rows, so the next pass should resume from that work once the FireOS/Vega runtime support from #162 has landed on main.