mobile: Support IME composition, file picking and native touch gestures

[huacnlee]

Note

Stack 5/13, ported from longbridge/gpui-mobile, where each layer was reviewed and merged as its own PR.
This PR is based on #51. Cross-fork PRs can't use GitHub's stacked PRs or target another PR's branch, so this diff includes every layer below it. Please review only the top commit (b5fed30), and merge the stack bottom-up.

1. build: Use gpui-pre 0.3.6 from crates.io so the crate can be published #48
2. ios: Support embedding GPUI in a native UIKit container #49
3. ios: Clear touch hover and preserve cancellation #50
4. android: Clear touch hover and preserve cancellation #51
5. 👉 mobile: Support IME composition, file picking and native touch gestures #53
6. mobile: Render frames on demand so an idle screen costs no CPU #55
7. ios: Keep a held Backspace repeating on the software keyboard #56
8. android: Add a host-driven entry point that survives Activity recreation #57
9. android: Pace frames to vsync and sleep between them #58
10. ios: Pin the Metal layer's contents instead of stretching them #59
11. android: Use Android's scroll physics and let hosts register an AssetSource #60
12. mobile: Keep a touch that stops a fling from inheriting the fling's axis #61
13. ios: Deregister IosWindow from the FFI window list on drop #63

Summary

iOS and Android Chinese keyboards could not reliably compose text, Add files returned an error or no result, and converting every drag to scroll prevented controls from claiming touch gestures.

• Use a native UITextView composition buffer to provide UITextInput support and forward plain and attributed marked text, UTF-16 selection, commits, and deletion to GPUI.
• Present an asynchronous UIDocumentPicker, returning sandbox copies of selected files (including images in Files), with multi-selection and cancellation support.
• Forward UIKit contacts as GPUI TouchEvents with stable IDs. GPUI's existing recognizer handles taps, long presses, control drags, scrolling, and inertia, replacing the duplicate iOS mouse/scroll synthesis.
• Add an Android InputConnection bridge with native-thread composition delivery, UTF-16 selection, Unicode-safe deletion, and session checks for stale callbacks. Keep legacy TextInput commit/backspace callbacks working. Custom hosts use GpuiInputActivity; the example inherits it.
• Connect Android file prompts to SAF and copy content URIs into app cache on a worker before returning readable paths. Register the picker Activity in the example.
• Forward Android contacts as TouchEvents, preserve multiple pointer identities, and distinguish cancellation from release. Remove duplicate mouse/momentum synthesis, desktop Fontconfig loading on Android, and unconditional frame presentation (matching the existing ai-chat host workarounds).
• Fix two simulator Debug crashes: UIKit dynamically forwards public keyboard trait setters, and NSString byte initializers require const void * rather than byte-pointer encoding. Correct the matching URL/maps initializer calls too.

This uses existing gpui-pre APIs; no GPUI core changes. TextView selection and scrollbar thumb dragging also require the companion gpui-kit handlers. The file pickers accept images accessible through Files/Documents; they do not add a separate Photos library entry point.

Touch-driven focus changes finish the current composition before dispatch. On iOS, programmatic focus switches without a touch or keyboard hide still need a platform focus identity signal; clearing the previous GPUI field's marked state on programmatic blur remains a follow-up.

Test Plan

• Passed: cargo check --target aarch64-apple-ios --lib --offline --no-default-features --features camera,video_player.

• Passed: cargo check --target aarch64-apple-ios-sim --lib --offline --no-default-features --features camera,video_player.

• Passed: Android backend library/test cross-compilation with NDK 27, and the complete ai-chat Android host's cargo ndk -t arm64-v8a --platform 26 check --locked --offline using these local changes.

• Passed: Android API 34 Java compilation, including the host's :app:compileDebugJavaWithJavac task.

• Passed: 2 host-run regression tests for the actual Unicode deletion and legacy callback helpers; Android touch regression tests compile but were not run on a device.

• Passed: Rust formatting and git diff --check.

• Passed on iOS 26.5 simulator Debug: input focus and English input; software Pinyin ni followed by selecting 你 inserts Chinese into the GPUI input; explicit keyboard dismissal; Files picker opens and cancels twice without crashing or reopening the keyboard.

• Passed: native objc2 Debug probe reproduces the old NSString signature rejection and verifies corrected English, Chinese/emoji, Chinese URL and empty-string roundtrips. Keyboard-trait forwarding was also checked against this UIKit runtime.

• Passed: attributed-marked-text runtime probe reaches the override once, preserves ni composition and UTF-16 selection, then commits Chinese. Requester confirmed Chinese input and subsequent iOS/Android device testing complete.

• Passed: full ai-chat Android Debug and optimized Release builds, installation and startup on Samsung Android 12 ARM64; Release package verified non-debuggable.

• Not addressed by a dedicated fix: the reported held-key repeat case (discrete 256 insert/delete callbacks pass), or the separately identified insertText:alternatives:style: entry point. No system autorepeat validation is claimed by those discrete-callback tests.

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