Add collapsible left navigation to prepaint APK

[isomorphisms]

Implements the navigation behavior from #50 on the packageable Android prepaint line, stacked on #25.

• wraps the existing prepaint screen in a real left navigation rail;
• moves the existing Open and Replay controls into that rail rather than duplicating actions;
• shows < while open and > while closed, with matching accessibility labels;
• uses a 150 ms width transition when Android animations are enabled;
• uses immediate state changes when system animations are disabled/reduced;
• cancels an in-flight transition before reversing, so rapid toggles converge on the latest requested state;
• collapses the rail to zero width / GONE rather than leaving a gutter;
• preserves the existing page, scroll, search, Termux/ICU, and prepaint state because the content view is reparented rather than recreated.

Backend boundary

This exact UI head has now been exercised on the physical phone. The rail toggle, file picker, Replay, page replacement, retained search text, and scroll/page-state boundary all behave as intended.

This PR is UI-only and must not choose an unfinished compiler backend. Its #25 base is now a transitional live-navigation harness. Current Android application integration targets direct DEX with JNI as the narrow native/platform boundary; ARM/Thumb and the experimental ARMv7 Termux compiler remain separate compiler-development work and are not dependencies of this APK line.

No source change is made here for that backend decision, so the tested UI head remains 672fc7febf9a48de3270ac3d8cee6ed54d2a26d1.