Add riscv64 (linux_riscv64) wheel to PyPI releases

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## Add `linux_riscv64` wheel to PyPI releases

### Summary

`pip install zstandard` on riscv64 Linux currently requires building from source (~3 min on a 1.6 GHz RISC-V SoC). Adding `linux_riscv64` to the wheel build matrix would give riscv64 users a prebuilt wheel.

### Evidence

| | |
|---|---|
| **Tested wheel** | [`zstandard-0.25.0-cp313-cp313-linux_riscv64.whl`](https://github.com/gounthar/riscv64-python-wheels/releases/download/v2026.03.11-cp313/zstandard-0.25.0-cp313-cp313-linux_riscv64.whl) |
| **Hardware** | BananaPi F3 (SpacemiT K1, rv64imafdcv, 8 cores @ 1.6 GHz, 16 GB RAM) |
| **Python** | 3.13 (CPython) |
| **Build system** | cibuildwheel |
| **Build time** | ~3 min (native, on hardware) |

The wheel imports and passes basic smoke tests on riscv64 Linux.

### Suggested CI change

**Workflow**: `.github/workflows/wheel.yml`

Add `riscv64` to the linux arch matrix and extend the QEMU condition:

```yaml
# In strategy.matrix.arch, add:
arch:
- 'aarch64'
- 'i686'
- 'ppc64le'
- 's390x'
- 'x86_64'
- 'riscv64' # <-- add this

# Update the QEMU step condition:
- name: Set up QEMU
if: ${{ matrix.arch == 'ppc64le' || matrix.arch == 's390x' || matrix.arch == 'riscv64' }}
uses: docker/setup-qemu-action@...
```

Add exclusions for non-existent images:
```yaml
exclude:
- { arch: 'riscv64', image: 'manylinux2014' }
- { arch: 'riscv64', image: 'musllinux_1_1' }
```

### Ecosystem context

- `manylinux_2_39_riscv64` images are available on `quay.io/pypa/` (landed in pypa/manylinux, 2025)
- cibuildwheel 3.x supports riscv64 via QEMU emulation
- auditwheel supports riscv64 platform tags
- maturin-action supports `riscv64gc-unknown-linux-gnu` cross-compilation
- Packages already shipping riscv64 wheels on PyPI: aiohttp, yarl, regex, markupsafe, charset-normalizer, rpds-py, multidict, propcache, watchfiles, rignore, setproctitle
- RISC-V hardware is shipping: SiFive HiFive, SpacemiT K1/K3, Sophgo SG2044 (64-core)

### Our temporary index

While upstream support is pending, we maintain a [PEP 503 index](https://gounthar.github.io/riscv64-python-wheels/simple/) with 50+ riscv64 wheels for the Python ML/AI stack, built natively on RISC-V hardware.

Tracking repo: https://github.com/gounthar/riscv64-python-wheels

Start with .github/workflows/wheel.yml and inspect the existing architecture matrix, QEMU setup, and image exclusions. Extend the riscv64 entries as described, run the workflow, and verify that a linux_riscv64 wheel is built and released to PyPI with the existing smoke tests passing.

Written by the indexing model from the issue text.