feat(agc): emulate undithered alpha-to-coverage - #3228

What

God of War Sons of Sparta (PPSA28997) stopped a few minutes into play with alpha-to-coverage is unsupported (register 0x2dc = 0x0000aa01).

• Decode. DB_ALPHA_TO_MASK = 0xaa01 is accepted: the enable bit with the four quad offsets at 2 and no OFFSET_ROUND, the undithered setting radeonsi programs. The following still throw:
  • any other offsets, or OFFSET_ROUND;
  • an MRT0 that is missing, swizzled, packed or exported without a floating-point alpha;
  • a pixel shader that exports a sample mask.
• Shader. Draws are single-sampled, so the pixel stage gets an alphaToCoverage flag. Such a variant declares SampleMask and starts with every sample covered. At its MRT0 export it writes 0 when the alpha is below one half or NaN, and all ones otherwise. It runs with late fragment tests. The shader keeps running after the export, so later stores still happen, as they do with a zero coverage mask on the GPU.
• Shader identity. The flag is part of the static shader identity (CacheKey) and of the request serialization, now version 16.
• Vulkan's alphaToCoverageEnable is not used: the NVIDIA driver 615.78.08 dithers single-sample coverage. An alpha of 0.25 writes half the pixels of an RTX 3050 Laptop target.

Tested

• The rule was measured on a Radeon 660M (gfx1035) with RADV and RADV_DEBUG=noatocdithering, which programs DB_ALPHA_TO_MASK = 0xaa01. I compared the GPU's own Vulkan alpha-to-coverage with this emulation for 16 alpha values: 0, -0, 0.25, the two floats below one half, one half, the two floats above it, 0.75, 1, 2, -1, both infinities and both NaNs. All 16 matched on the 660M, and the emulation gave the same result on the RTX 3050. The rows are pinned in tests/execution/AlphaToCoverage.cpp. They are not measured on gfx1013, as TechnicalDebt.md says.
• Arch Linux, kernel 7.2.9, GCC 16.2.1, Release, on main d31b57a: ctest 752/753 passed on the RTX 3050. The failure is agc_driver_dcc_keys_first_write, which fails on main on this GPU too. Each commit builds and passes the tests it touches (agc_shader_memory, agc_driver_graphics, agc_driver_dual_source_blend).
• PPSA28997 now plays past that point, through several levels, with feat(libkernel): implement _umtx_op wait and wake operations #1482 and feat(libkernel): deliver sceKernelRaiseException on Linux #1431 applied for Linux.

Checklist

• Based on current main; no other open PR implements the same functions
• One topic per PR; follow-ups go in a new PR
• No comments in code except technical debt
• Unimplemented paths throw (NotImplemented_nid_no_patch); silent stubs are listed in TechnicalDebt
• No notes, investigation .md files or images added to the repository (attach them to this PR)
• New third-party code is a submodule built from source
• Generic behaviour, not specific to one title
• Depends on: none
• AI-assisted: yes

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