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docs(research): identify firmware exit layout as BIOS readback mismatch

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Compare the current trace with GBATEK, the pinned public test, Smolka firmware analysis, and mGBA software BIOS behavior. Distinguish generic image-derived protection from replacement firmware compatibility. Plan guarded PC+8 compatibility data in the generated BIOS instead of test-specific bus overrides.
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docs/references.md
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@ -85,6 +85,16 @@ Hardware references used for the core:
- [mGBA Thumb transfer implementation](https://github.com/mgba-emu/mgba/blob/master/src/arm/isa-thumb.c) and [shared memory transfers](https://github.com/mgba-emu/mgba/blob/master/src/gba/memory.c), reviewed for empty STM/PUSH routing and the extra Thumb instruction width on the stored pipeline PC. This supports executing PC+6, rather than an ordinary PC+4 operand read.
- [GBATEK Thumb jumps and calls](https://problemkaputt.de/gbatek-thumb-opcodes-jumps-and-calls.htm).
BIOS protected-read root-cause research:
- [Smolka Progress Report #6](https://www.smolka.dev/posts/progress-report-6), for firmware-dependent protected reads and the Ruby/Sapphire null-pointer case; the report distinguishes Emerald.
- [mGBA: Cracking the GBA BIOS](https://mgba.io/2017/06/30/cracking-gba-bios/), for BIOS access protection and replacement-versus-external firmware approaches.
- [Cult-of-GBA BIOS](https://github.com/Cult-of-GBA/BIOS/tree/a30e9a96df083628b650724b7d4d7112b4070b98), reviewed for original boot, SWI, and IRQ exit layout and its MIT license. No source or binary was copied.
- [mGBA BIOS implementation](https://github.com/mgba-emu/mgba/blob/master/src/gba/bios.c), for the explicit post-SWI `biosPrefetch` compatibility value.
- [no$gba BIOS FAQ](https://problemkaputt.de/gbabios.htm), for the distinction between simulated firmware services and using a firmware image.
See [the root-cause analysis and implementation decision](research/bios-readback.md).
Use [GBATEK](https://problemkaputt.de/gbatek.htm) and ARM7TDMI documentation for further hardware work.
Only use game ROMs that you may lawfully use. Do not commit game ROMs, BIOS files, or game assets.
The ignore file excludes `roms/`, common game ROM extensions, and save files.