Windows AMD (ROCm) stack switching: stabilization follow-ups

Follow-up from #1248. The PR description says Windows ROCm switching "stays gated until AMD's standard install path stabilizes", but that is now stale: the production torch-index manifest serves two `amd-multi-arch-index` entries targeting `win32` (torch 2.10.0 and 2.11.0 on `rocm7.14.0`), and the code explicitly supports that mechanism on Windows (`torchIndexManifest.ts` allows only `amd-multi-arch-index` AMD entries on win32; the acquisition URL is the hardcoded `AMD_MULTI_ARCH_INDEX_URL`). Windows AMD stack switching and snapshot restore of these stacks were validated end-to-end on real hardware (W7800/gfx1100 and RX 7600/gfx1102).

We want to keep facilitating AMD's new install method (TheRock multi-arch index) in Desktop. Remaining items to resolve for a future release:

1. **Correct the stale wording.** Update the PR description / `docs/torch-index-stacks.md` so nobody concludes Windows ROCm switching is unsupported.
2. **Track AMD install-path stability.** The Windows distribution is still nonstandard: AMD-hosted wheels, `torch[device-all]` extras, per-arch device overlays (`amd-torch-device-gfx*`), and a distinct versioning scheme (`+rocm7.14.0`). If AMD changes the index layout, overlay naming, or moves to pytorch.org, the hardcoded mechanism needs a coordinated update (older desktops safely drop unknown manifest kinds).
3. **Built-in fallback manifest has no AMD entries.** `INDEX_STACKS` contains only cu126/cu128, so a `win-amd` install with no network and no last-good manifest cache sees no switching options at all. Decide whether a baseline AMD entry should ship in the fallback once AMD's path is stable.
4. **Hardware coverage.** Validation so far covers gfx1100/gfx1102 (RDNA3/Navi33). Broader AMD architecture coverage (and the rocSHMEM/libnuma startup noise seen on Linux with ROCm 7.14) should be understood before recommending these stacks broadly.

Start with docs/torch-index-stacks.md, torchIndexManifest.ts, INDEX_STACKS, and the AMD_MULTI_ARCH_INDEX_URL path. Review the existing Windows AMD manifest handling and the validated W7800/RX 7600 cases before assessing the fallback-manifest and broader hardware questions. Done means the documentation is accurate and the release decision, coverage, and any required follow-up are recorded.

Written by the indexing model from the issue text.