sqlglotc: add build-sqlglotc.yml for riscv64 wheels

[luhenry]

• Package: sqlglotc
• Version: 30.17.0
• Source: https://github.com/tobymao/sqlglot
• Docs: https://sqlglot.com/

Mypyc-compiled companion extension for sqlglot: cibuildwheel builds the sqlglotc/ subdirectory of the tobymao/sqlglot monorepo, compiling a subset of sqlglot's own modules into .so files that shadow the pure-Python ones once installed. Upstream publishes no riscv64 wheel.

Mirrors upstream's package-publish.yml build-wheels job.

Differs from upstream

• Nothing beyond the riscv64 image - sqlglotc/pyproject.toml carries no [tool.cibuildwheel] table to inherit from, so CIBW_MANYLINUX_RISCV64_IMAGE is the only build-side addition.

Matrix: cp312/cp313/cp314, no cp314t - upstream itself ships none (mypyc doesn't support free-threaded builds).

Testing

• Upstream's build-wheels job runs no tests; added CIBW_TEST_COMMAND running upstream's own make unitc recipe (SKIP_INTEGRATION=1 python -m unittest) against the installed compiled wheel.
• duckdb/pandas (imported by tests/test_optimizer.py and tests/test_executor.py) come from our registry - no riscv64 wheels on public PyPI.

License: OK

CI running on riscv64; will update with final pass/skip counts once the run completes.