[FEP-0034] RISC-V Experimental Build Matrix Tracking

## Summary

Tracking issue for FEP-0034 (Experimental RISC-V Support, #34) — the authoritative tracker referenced by the FEP's Related PRs section.

Two phases run in parallel per the approved FEP:

1. **Compile adaptation** — attempt a `riscv64` build per module; land build helpers + CI matrix entries, or a documented not-yet-supported status, in the originating repository.
2. **Dependency analysis** — classify deps for RISC-V availability; land trimming/relaxation PRs in the originating repositories.

Each module ends in one of three statuses: **Compile-adapted** / **Partially adapted** / **Not yet supported**. Per the FEP, every module succeeding is *not* a requirement — a documented not-yet-supported status is a valid outcome.

**Milestone:** none. `Target Version` stays `FlagOS TBD` — appropriate for an experimental track whose outcome (including "not yet supported") is itself the deliverable. Build helpers and compile evidence feed into sig-os (FEP-0019) for any future distribution decision.

## Build matrix — priority subset (0/4)

- [ ] FlagCX — `riscv64` build experiment — PR: TBD — status: TBD
- [ ] FlagTree — `riscv64` build experiment — PR: TBD — status: TBD
- [ ] FlagGems — `riscv64` build experiment (architecture-level; vendor backends tracked under FEP-0033 / #33) — PR: TBD — status: TBD
- [ ] FlagScale — `riscv64` build experiment — PR: TBD — status: TBD

## Build matrix — remaining Wave 1 modules (join as the matrix progresses)

- [ ] FlagQuantum — status: TBD
- [ ] FlagTensor — status: TBD
- [ ] FlagAudio — status: TBD
- [ ] FlagBLAS — status: TBD
- [ ] FlagDNN — status: TBD
- [ ] FlagAttention — status: TBD
- [ ] FlagSparse — status: TBD

## Dependency-trimming PRs

Populated as the dependency analysis lands (classification: keep / block-and-document / mark-optional / build-only / remove).

- (none yet)

## Exit criteria

- Every in-scope module covered: either `riscv64` enablement merged, or not-yet-supported status documented in the originating repository (README note, `ARCH_SUPPORT`-style file, or CI skip)
- Smoke test done for each module whose build succeeds (install-layout-level check on `riscv64`)
- Then: open a PR flipping FEP-0034 Status from `Implementable` to `Implemented`

## Coordination

- Vendor-specific RISC-V backends (e.g. SpacemiT's FlagGems backend, FEP-0033 / #33) are consumed, not duplicated, by this track.
- Distribution-level packaging, distribution matrix, and publishing channels belong to sig-os (FEP-0019, #19).

/cc @wbavon

Start by reading FEP-0034 and choose one priority module: FlagCX, FlagTree, FlagGems, or FlagScale. Inspect that module's build configuration and CI entry points in its originating repository, then record either successful riscv64 compilation with a smoke test or a documented not-yet-supported status. The issue is complete when every in-scope module has one of those outcomes and the FEP status can be updated.

Written by the indexing model from the issue text.