Added: Add basic kitty keyboard protocol disambiguation support

[JM-FRANK]

Termux currently sends overlapping legacy encodings for Ctrl+I/Tab and unmodified/modified Enter. This adds negotiated kitty keyboard disambiguation (flag 1), so applications can distinguish these combinations, Ctrl+digits such as Ctrl+0, and Escape.

Refs #5300 and #5198.

Changes

• Handle CSI ? u, CSI > flags u, CSI < count u, and CSI = flags ; mode u, with separate bounded mode stacks for the main and alternate screens and reset handling.
• Encode modified text keys using their unshifted layout character; add canonical functional/keypad encodings, including modified Enter/Tab and the non-conflicting F3 encoding.
• Connect hardware input, soft input/extra modifiers, and existing virtual Fn mappings to the negotiated mode. Preserve legacy input when the mode is disabled and retain plain Enter/Tab/Backspace recovery bytes.
• Add parser/encoding regression tests, Android input-path tests, and validation documentation with expected/actual device bytes.

Review fixes and current validation

• Resolve printable Space through the active Android layout before encoding, preserving AltGr layout text while retaining Ctrl/left-Alt+Space shortcuts.
• Restore existing Shift+PageUp/PageDown scroll-action precedence; ordinary/Ctrl+Page keys still reach the application.
• Centralize functional-key mode selection in KeyHandler.getCode for view and virtual Fn consumers.
• The branch is consolidated into one feature commit following the repository convention. Squashing preserved the complete file tree; validation reports retain the original tested source identifiers.
• 150 terminal-emulator tests and 10 app tests passed locally (160 total, zero failures/errors/skips). Four new input regressions cover AltGr layout characters, Ctrl/left-Alt+Space, scrollback precedence in both modes, and other Page-key delivery. AltGr uses a synthetic layout fixture.
• Current fix source: 65c625b2. Complete Debug APK assembly passed for arm64-v8a, armeabi-v7a, x86, x86_64 and universal output. APKs, checksums, local test reports and a device-retest checklist are retained locally.
• Review-fix APK 0.118.0+kitty.65c625b2 is now installed on the Android 16 arm64 test phone. All 96 recorded checks passed: 83 Android framework KeyEvents delivered directly to production TerminalView/native PTY with scroll checks, 2 checks of the current virtual AltGr+Space mapping (no character), and 11 real Rime touch/modifier/lifecycle checks. The direct framework-event checks do not cover physical HID or OS input dispatch.
• The user reports completing manual combination retests on review-fix source 65c625b2, across on-device/external physical keyboards, local Termux/SSH, and Zellij/tmux. This is user-reported verification; versions and raw captures were not supplied.
• Current virtual AltGr+Space maps to no character. An external layout producing NBSP or another character remains unverified; the combination retest report does not specifically confirm this layout case.
• Review-fix device methods and exact byte results.
• git diff --check passed.

Historical validation before review fixes

• 150 terminal-emulator tests and 6 app tests passed, including 4 Robolectric input-path tests.
• Debug APK assembly passed for arm64-v8a, armeabi-v7a, x86, x86_64 and universal output.
• On a physical Android 16 arm64 device, 52 comparisons passed using system-injected key events through the installed Termux, native PTY and foreground shell capture. Mode queries also checked stack restoration and main/alternate-screen isolation.
• 7 additional device checks passed using real Rime keyboard touches and the Termux CTRL extra key, including app background/foreground and keyboard close/open.
• The user reports successful manual tests combining on-device/external physical keyboards, local Termux/SSH, and Zellij/tmux, with kitty-encoded input reaching the receiver. Versions and raw captures for these manual tests were not supplied.
• Runtime validation above used pre-fix source commit 91ae4885. It does not validate the review fixes below.

Protocol scope and validation records.

Protocol scope and limitations

Only disambiguation flag 1 is implemented. Repeat/release events (2), alternate key codes (4), reporting all keys (8), and associated text (16) are not implemented; unsupported flags are masked out and are not advertised as enabled. This PR references the feature requests without closing the full-protocol request.

Text-only IME commits cannot preserve physical modifier/key identity that Android does not deliver. Termux application shortcuts retain precedence over terminal input.

Known limitation deferred to future work: virtual Fn (volume-up shortcut) mappings do not preserve Ctrl on the resulting key. For example, Ctrl+virtual Fn+W sends an unmodified Up rather than Ctrl+Up. That combination is not covered by the recorded passing checks.