#1796 Docs: on xe there is no SSD expert tier, and --spec 0 is refused for native packs, but the Intel docs and TROUBLESHOOTING say otherwise

Descripción

**Environment**
- Ubuntu 26.04.1, kernel 7.0.0-34, 2× Intel Arc Pro B70 (`xe`)
- compute-runtime 26.31.39395.14, oneAPI DPC++ 2026.1.1
- Native build (no Docker), AOT `-DSTRATA_SYCL_AOT=bmg-g31`
- Strata `fb58e0d` (v0.1.41)
- Config as `setup.sh --backend sycl` writes it for a B70: `--stream-experts`, with `STRATA_VERIFY_NO_HOST=1` from the wrapper

**Summary.** There are two engine behaviours a B70 user hits that the docs either leave out or contradict.

### a. With `STRATA_VERIFY_NO_HOST=1`, any expert that is neither in VRAM nor in the mirror makes the engine refuse to start

- Engine: `sycl/src/program/generate.cpp:4736-4741`
  ```
  strata generate: REFUSED: %lld experts are neither in VRAM nor mirrored; with STRATA_VERIFY_NO_HOST the device plan
  cannot run them and generation would lack a safe host fallback - raise STRATA_MIRROR_MIB or the free RAM, or lower --max-context
  ```
  We saw `8290 experts are neither in VRAM nor mirrored ...` when we capped VRAM and the mirror to test an SSD tier.
- `docs/INTEL_ARC.md:152-154` says `STRATA_VERIFY_NO_HOST=1` "is needed on `xe` cards when part of the experts is
  not in VRAM", but not that the experts must then all be in VRAM **or the pinned mirror**, with no SSD fallback.
- `docs/INTEL.md:394-395` **contradicts the engine**: "with `--stream-experts` there is no host copy of the experts,
  so any expert left out of VRAM is read from the SSD and computed on the CPU whenever it is routed". With the
  wrapper's default (`docs/INTEL.md:72-75`) the engine refuses instead. This text looks older than the 4736 guard.
- The only correct description is in the A750 section (`docs/INTEL.md:693-702`), which a B70 user has no reason to read.

**Expected (docs):** a short statement in the B70 / `xe` part of `INTEL.md` and in `INTEL_ARC.md`, something like:
"On `xe` (`STRATA_VERIFY_NO_HOST=1`) every expert must be in VRAM or in the pinned RAM mirror
(`STRATA_MIRROR_MIB`). There is no SSD expert tier; if the two do not cover the model, the engine refuses to start.
Size RAM accordingly, or lower `--max-context`." Also fix or remove `INTEL.md:394-395`.

### b. `--spec 0` (or 1) is refused for native (IQ) packs

- Engine: `sycl/src/program/generate.cpp:2630-2635` (and upstream `src/program/generate.cpp:2587`):
  ```
  strata generate: %s is a native (IQ) pack: it needs --native SHARD1, --spec T (T >= 2) and --prefill CHUNK
  ```
- `docs/TROUBLESHOOTING.md:124` tells users that "`--spec 0` avoids the verify window while you wait for a driver
  fix". For a native pack that advice ends in this refusal. Neither `INTEL.md` nor `INTEL_ARC.md` says that
  speculative decoding cannot be turned off for native packs, so "spec on vs off" cannot be A/B tested on them.

**Expected (docs):** state the `--spec >= 2` requirement for native packs next to the `--spec` examples in
`INTEL.md` (e.g. near `:208-220`, `:293`). In `TROUBLESHOOTING.md:124`, give the workaround that does work for native
packs (`--spec 2`, the smallest window?), or say that `--spec 0` applies only to non-native packs.

**Suggested fix:** docs only. Optionally, make the 2632 message say *why* T must be at least 2, and the 4738 message
say "there is no SSD fallback on this path", so the error itself points users to the docs.

**Related (not duplicates):** open #1440 (2× B70, `--layer-split`) hits the same `REFUSED: ... neither in VRAM nor
mirrored` line. There the cause is a mirror bug on a split; here it is the documented single-card tier rule. Linking
both helps the docs fix land in the right place. Open PR #1390 edits `docs/INTEL.md` (Windows), so expect a
conflict there. Nothing found on `--spec 0` for native packs.