#2153 docs: the xe expert tier has no SSD fallback, and --spec 0 is refused on native packs (#1796)

Beschreibung

Issue: Resolves #1796

## Summary

Docs corrections for Intel Arc (`xe`) users: the expert tier under `--stream-experts` has no SSD fallback (the engine refuses to start), and `--spec 0` is refused on native (IQ) packs.

## What changed

- `docs/INTEL.md`: replaced the stale "read from the SSD" claim in the B70 measurement section with the actual rule (every expert in VRAM or the pinned mirror, otherwise refusal at startup); added the `--spec >= 2` requirement for native packs next to the first `--spec` table.
- `docs/INTEL_ARC.md`: added the same VRAM-or-mirror / no-SSD-tier rule to the VRAM bullet.
- `docs/TROUBLESHOOTING.md`: noted that the `--spec 0` workaround does not apply to native (IQ) packs.

## Extra Notes

- Both engine behaviors verified against the current code: `REFUSED: %lld experts are neither in VRAM nor mirrored` (with `STRATA_VERIFY_NO_HOST`) in `sycl/src/program/generate.cpp`, and `%s is a native (IQ) pack: it needs --native SHARD1, --spec T (T >= 2) ...` in the same file.
- The line numbers in the issue have shifted (the code moved), so I located the guards by their message text.
- Docs only; no engine or behavior changes.