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feat(input): implement KEYCNT keypad interrupt control

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Add a dependency-free keypad state machine in gba-core, map KEYCNT, latch IF bit 12, and wake HALT through the existing interrupt gates. Sample complete CPU/DMA control writes once so transient byte values cannot create false requests.

Document the mGBA-style polling policy and its unverified hardware retrigger limits. Cover every button-state/selection pair, register masks, repeated requests, byte/word writes, failed-store isolation, DMA priority, ARM/Thumb IRQ return, BIOS IntrWait, and retained BIOS reset state.

Follow the restructured workspace and modular documentation. Validated on Darwin arm64: workspace and core/demo tests pass in debug and release, with Clippy, rustdoc, ten native window smoke tests, and terminal demos passing.
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0xrsydn 2026-10-05 18:00:46 +07:00
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docs/references.md
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@ -45,7 +45,11 @@ Hardware references used for the core:
- [NanoBoyAdvance composition](https://github.com/nba-emu/NanoBoyAdvance/blob/master/src/nba/src/hw/ppu/merge.cc), reviewed for palette lookup and horizontal mosaic after sprite preparation.
- [mGBA window renderer](https://github.com/mgba-emu/mgba/blob/master/src/gba/renderers/video-software.c), reviewed for inverted bounds and vertical edge flags.
- [NanoBoyAdvance window implementation](https://github.com/nba-emu/NanoBoyAdvance/blob/master/src/nba/src/hw/ppu/window.cc), reviewed for persistent vertical flags, four-cycle horizontal comparisons, offscreen columns, and end-edge precedence.
- [GBATEK keypad input](https://problemkaputt.de/gbatek-gba-keypad-input.htm).
- [GBATEK keypad input](https://problemkaputt.de/gbatek-gba-keypad-input.htm), for KEYINPUT/KEYCNT fields and selected-button OR/AND matching.
- [mGBA keypad sampling](https://github.com/mgba-emu/mgba/blob/master/src/gba/gba.c) and [control writes](https://github.com/mgba-emu/mgba/blob/master/src/gba/io.c), reviewed for repeated OR requests, AND snapshot suppression, and newly selected held keys.
- [NanoBoyAdvance keypad implementation](https://github.com/nba-emu/NanoBoyAdvance/blob/master/src/nba/src/hw/keypad/keypad.cc), compared for control access widths; its AND/retrigger behavior differs from the chosen polling model.
- [mGBA keypad timing issue](https://github.com/mgba-emu/mgba/issues/2490), for frontend frame-based input timing limits.
- [Tonc hardware interrupts](https://gbadev.net/tonc/interrupts.html), for keypad source enable and IF/IE bit 12.
- [GBATEK memory mirrors and video byte writes](https://problemkaputt.de/gbatek-gba-unpredictable-things.htm).
- [GBATEK GBA memory map and bus widths](https://problemkaputt.de/gbatek-gba-memory-map.htm).
- [GBATEK GBA DMA transfers](https://problemkaputt.de/gbatek-gba-dma-transfers.htm).