[bluetoe-99]

Why. A storage buffer bound for writing is recorded as its whole bound range, and any texture whose storage overlaps that range is then re-uploaded whole. In Castle Town one of the game's compute pipelines binds a 1.75 MB write buffer whose range lands inside a finalized 2560x1440 render target's storage, and every frame of that burst the renderer copied 15 MB of guest memory over the image, on a path where nothing at all had been written by the CPU (upload over a drawn image ... 0 KB dirty in 0 ranges, a GPU buffer wrote it). This is one of the costs measured under #2780, where the resident/mirror bookkeeping grows with each area change.

What changes.

• gpu_wrote_into reports the ranges it matched, clipped to the texture's storage and merged, in image-relative offsets.
• texture_touch uploads those ranges instead of the whole image, and the CPU's writes since the last upload in the same pass. That second half is required: the whole upload used to cover those pages, and the upload moves upload_gen past them, so a partial that skipped them would lose them permanently. An event where the CPU also wrote still uploads the full amount.
• Sparse images keep the whole path; RX_VK_NO_PARTIAL and RX_VK_GPU_WRITE_WHOLE=1 restore the old behaviour for A/B.
• A tally on the path, so anyone can see whether it is hot in their own scene rather than guessing: vk: gpu-write uploads: N (X MB copied, the whole image would have been Y MB).

Measured (Linux, RTX 5070 Ti 16 GB, one save, one process, five area changes: Hyrule Field, Castle Town Market, Kokiri Forest, Hyrule Field, Castle Town Market). Both arms were built from the same tree with the same flags, differing only by this patch, and the tour was run twice per build:

	stock	patched
per bind-only write-back event	15,360 KB (whole image)	1,800 KB (the bound range)
event where the CPU had also written	15,360 KB	15,360 KB (the union, by design)
the path over a tour, sample 1	150 events, 1,462 MB	150 events, 648 MB
the path over a tour, sample 2	150 events, 1,434 MB	150 events, 632 MB
retirement past the budget, last leg	2,910 textures (841 retire lines)	3,264 textures (840 lines): no eviction regression
fps leg medians	91.8/94.0/96.0/81.9/90.3, then 91.4/88.9/90.1/93.1/88.2	88.0/92.5/77.1/85.5/87.4, then 91.3/90.5/91.5/92.4/92.0

So this is a copy and staging reduction on a path that arrives in bursts, not a frame-rate win on a 16 GB card: 56% less copied on the path, 802-814 MB not copied per tour. No fps effect is claimed, and none was measurable. On the 8-12 GB cards of #2690 and #2780 the same events compete with a texture budget that is already being evicted past, which is where the saved copying should matter more.

Cache cost for players: none. nx2/build.py's HOOKSHOT_SHADER_STAMP hashes the translator sources (eden/, sirit/, compat/, vk_shader.cpp, vk_bank.cpp, vk_state.h, vk_api.h) and deliberately not vk_texture.cpp, so existing translated programs, pipeline notes and hookshot_prepared.bin stay valid after this ships.

Tests. tests/test_gpu_write_part.py (new) cuts the recorder and the reporter, texture_gpu_wrote and gpu_wrote_into, out of the source and runs them against stand-ins: a write inside reports its part; one spanning either end is clipped; adjacent writes merge; an unfinished submission is not reported until it completes; the same write is not reported to the same texture twice; a write elsewhere reports nothing. Plus source guards for the switch, the CPU-write union and the sparse exclusion. It compiles its fixture with -D_CRT_SECURE_NO_WARNINGS, the same flag the sibling tests pass for the MSVC CRT's getenv deprecation. python -I -B tests/run_all.py with the Vulkan and SPIR-V headers available: 492 passed, 0 failed, 35 skipped on 9b6072cd (the skips want spirv-val and a few Windows-only tools), and 411 passed, 13 failed, 67 skipped on the older base the measurements were taken on, those 13 failing identically on the pristine checkout of that base.

Not claimed. No visual change is intended. The change copies a strict subset of what the old path copied, so the known hazards of partial upload (the rainbow-sky case, where an unrelated variable sharing an edge page re-uploaded garbage over a drawn LUT) get narrower, not wider.