fix(ci): name wheel artifacts by platform tag, not runner label

[philomath213]

The wheel artifact name used the runner label. Two matrix rows share ubuntu-24.04, so manylinux and musllinux both uploaded as dist-wheel-<py>-ubuntu-24.04. The uploads succeed. But download-artifact with merge-multiple extracts artifacts with the same name over each other, so the publish job ships one platform less and still reports success.

TestPyPI 0.3.17.dev6 shows the result: 13 files, not 17, and no manylinux wheels. The rehearsal before musllinux (dev4) also had 13 files, but with manylinux. The new platform replaced a platform instead of adding one.

• Name the artifact after matrix.target[1], the platform tag. It is unique for each row, and the job name and CIBW_BUILD already use it
• Correct the wheel counts in RELEASING.md from 12 to 16. That count is the check which must catch this problem

The matrix gives 16 artifact names, and all 16 are unique. The names stay unique with the manylinux_aarch64 row from #529: 20 names, 20 unique.

tests.yml cannot verify this fix, because it uploads no artifacts. Use a TestPyPI rehearsal. Expect 17 files, and manylinux_2_28_x86_64 must be present.

Released versions are correct. v0.3.17 is older than musllinux, so its 12 wheels are complete. Only a release after #528 loses manylinux.