#1109 docs(hip): GPU_PINNED_MIN_XFER_SIZE=1048576 stopped our gfx1030 mmap-experts stalls (#267, #649)

Descrição

Re-opens #920. The 2026-10-06 history cleanup closed that PR automatically, so this is the same docs commit cherry-picked onto the new `main`. One conflict was resolved: the new #835 FP16 bullet in `docs/AMD_HIP.md` stays, and the #267/#649 bullet sits after it.

## New: an A/B run on release v0.1.40.1

I removed the variable and ran the same box again (RX 6800 16 GB, gfx1030, ROCm 7.2.4, 31 GiB RAM, IQ3_XXS, `--mmap-experts`, unit with `MemoryMax=28G`, release v0.1.40.1 at 82f46a8; its engine reports 0.1.40 because v0.1.40.1 changed only the Python server). It used random prompts of 2k, 8k and 16k words, 400 tokens out each.

- 2k and 8k words: fine (`prompt 7983 tokens ... 209.7 tok/s, 400 generated ... 12.7 tok/s`).
- 16k words: stalled, and no request after it ran.

```
strata serve: no progress for 60 s during a request (reading the prompt (batched): layer 18 of the prompt chunk from token 0) - stopping the engine so the server starts it again (issue #29)
strata serve: stall report (engine 0.1.40): stage "reading the prompt (batched): layer 18 of the prompt chunk from token 0" for 818 s; 0 layers served since the last finished step (0 = stopped, more = slow)
  expert pool threads: w0=sleeping w1=sleeping w2=sleeping w3=sleeping w4=sleeping; host idle for 841520 ms
  threads waiting on the disk (state D): 0 of 31
  memory: 14180 MiB resident, 262 MiB in swap, 20852 MiB RAM available
```

GPU use was 0% and VRAM was full during the stall. The engine did not restart by itself after 818 s. I stopped the unit. With the variable set, this box has had zero stalls since 2026-10-04.

## The engine already has the cure, for one mode only

`src/program/generate.cpp:1515` sets `GPU_PINNED_MIN_XFER_SIZE=1000000` (MiB) when `STRATA_ARENA_MMAP=1`. The comment there names the same mechanism my first post guessed at: ROCclr locks pageable sources of 1 MiB or more in place as a GPU userptr. `--mmap-experts` does not set `STRATA_ARENA_MMAP`, so it does not get that default. My 1048576 has the same effect as your 1000000: both are far above any copy, so ROCclr always stages.

A one-line code follow-up is possible: also apply that default when `--mmap-experts` is on. I have not done it in this PR. It stays docs only unless you want the code change.

Related: #267, #649.

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