Make the engine platform-agnostic (after the CLI full-agnostic phase, cli#471) #902

Sequencing: do this after the CLI full-agnostic phase (labelle-cli#471) is done.

Goal: labelle-engine carries no platform-specific code; platform behaviour lives in the platform packages (labelle-android, labelle-ios, labelle-web) and the backends. Same principle as the CLI core, and as the iOS scaffolding removal already in progress.

Known Android code (engine main, 2026-09-28):

1. src/android.zig (~800 lines): Android immersive mode. JNI calls through the ANativeActivity (reached via labelle-core's backend-agnostic seam) hide the system bars, hook onWindowFocusChanged/onContentRectChanged, and fall back to the legacy decor-view API below API 30. It's wired from src/root.zig:41 and driven by the assembler's .android.immersive_mode key. → Move to labelle-android next to its runtime services (intent env, debuggable, relayout, AAudio, video), and have the engine or assembler call it through the plugin, not engine code.
2. src/preview_shm.zig:58: an is_android_abi branch in the editor-preview shared-memory code. → Make it target-generic, or move it behind the same seam.
3. src/storage/default.zig:54: an Android check for the default save-storage location. → Make it target-generic, or let the platform package supply the data directory.

To do when this starts:

• Run a full sweep for other platform-, backend- or store-specific code in the engine (src, build.zig, tests, CI) and list it.
• Consider an agnostic guard for the engine like the CLI's agnostic_guard_test.zig, with a shrink-only allowlist.
• Keep behaviour identical on Android. Flying Platform uses immersive mode, so the change needs on-device acceptance on the SM-T505.

https://claude.ai/code/session_01R2K5sJDL5Msn1uRTkybiDP