Merge dev into dual-ssd-mirror: resolve #298/#192-era conflicts

Combined resolution: mir_pread/st_prefetch_rep (this PR) now carry dev's
DISK-CLASS accounting unwind (dc_wall_exit) and O_DIRECT prefetch skip
(g_direct); direct-path keeps dc_direct=1 plus the mirror read counters.
Verified: clean build, token-exact tiny models unchanged, test_st_mirror
and test_st_pread pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/ENVIRONMENT.md

@@ -114,6 +114,15 @@ Per-drive byte counts are reported in a `MIRROR:` stats line. Combine with `DIRE
 | `COLI_CUDA_SHARED_W4A16_MIN_ROWS` | `32` | Min row count to engage the shared-MLP W4A16 kernel. |
 | `COLI_METAL_UNTRACKED` | off (Metal only) | `=1` sets `MTLResourceHazardTrackingModeUntracked` on Metal buffers (reduces hazard-tracking overhead). |
 
+> **Windows note.** On Windows, a bare `coli chat` / `coli run` / `coli serve`
+> (no `--gpu`/`--vram`/`--auto-tier`) **auto-enables the GPU** when it detects a
+> CUDA build (`coli_cuda.dll` next to the engine) and at least one GPU via
+> `nvidia-smi`. The expert-tier VRAM budget is then sized automatically from the
+> card's free VRAM (same computation as `--auto-tier`). If `nvidia-smi` is not on
+> `PATH` the run falls back to CPU with a warning — pass `--vram N` (or add
+> `nvidia-smi` to `PATH`) to enable CUDA in that case. `--gpu none` forces
+> CPU-only. (Linux/macOS behaviour is unchanged: pass a flag to enable CUDA.)
+
 ---
 
 ## Advanced / experimental / debug