feat(iap): add plugin native source support to Android build templates

[A-Legg]

feat(iap): add plugin native source support to Android build templates

This PR adds the template-level groundwork for mob_dev to auto-discover and compile plugin native C sources into Android builds.

Changes

AndroidManifest.xml.eex

Adds the com.android.vending.BILLING permission required by Google Play Billing (used by mob_iap and any other IAP plugin). Included unconditionally — it has no runtime effect if the app doesn't use IAP, and it avoids generator complexity.

build.zig.eex

Adds a new -Dproject_plugin_sources option that accepts comma-separated name:path pairs. When mob_dev scans dependencies and finds .c files under priv/native/android/jni/, it will pass them via this flag. The build.zig template compiles each entry as a plain C object and links it into lib<app>.so.

Coordination

This is the mob_new counterpart to the mob_dev plugin auto-discovery work and the mob runtime NIF hooks PR:

• mob runtime: feat: IAP runtime NIF hooks (StoreKit 2 + Play Billing 7) mob#27
• mob_iap plugin: https://github.com/A-Legg/mob_iap

Backwards compatibility

Projects without plugin native sources pass an empty string (the default), and the new iteration block is skipped. No effect on existing builds.

What's still needed in mob_dev

mob_dev's NativeBuild module needs to:

1. Scan deps/*/priv/native/android/jni/*.c before each Android zig build
2. Produce name:path pairs and pass -Dproject_plugin_sources=...
3. Copy plugin .kt files from priv/native/android/ into the project's Java tree
4. Scan deps/*/priv/native/ios/*.swift and pass -Dproject_swift_sources=... for iOS

These mob_dev changes are planned as a follow-up PR.

[A-Legg]

🚀 Test app now available: https://github.com/A-Legg/mob_iap_test

This repo demonstrates the full IAP flow end-to-end on both iOS Simulator and Android Emulator. It validates that:

• StoreKit 2 / Play Billing 7 callbacks reach the BEAM
• Mob.Test.assigns/1 shows :products state after fetch_products
• All 5 IAP NIFs (iap_fetch_products, iap_purchase, iap_restore, iap_current_entitlements, iap_manage_subscriptions) return :ok

See the README for step-by-step test instructions + troubleshooting.