Kill one writer, meet the next

#3342 stopped stamp-counts stamping the pinned entry. The release branch went red again anyway: build-plugins.sh carries its own version sync, which selects entries by plugin name and stamps them with the project version. The pinned entry is literally named ork, so the fresh-build step re-broke the checkout #3342 had just protected.

writer 1  stamp-counts   .plugins[0].version = $v          fixed #3342
writer 2  build sync     select(.name == "ork").version = $v  this PR

Also swept in: test-plugin-installation.sh validated versions against a bare X.Y.Z and called 10.0.0-alpha invalid — the third appearance of the regex fixed in #3337.

Run the sync on the real release-branch data

select(.name == $name)
| select((.source | type) != "object" or (.source.ref // "main") == "main")

An entry is stamped only when its ref tracks main. Pinned entries keep the version their tag agreement demands; string-source layouts behave exactly as before.

Measured

project version 10.0.0-alpha, marketplace from the RELEASE branch:
  old sync   ork 10.0.0-alpha pin broken   ork-alpha 10.0.0-alpha
  new sync   ork 9.8.0 pin kept          ork-alpha 10.0.0-alpha

semver regex (test-plugin-installation):
  9.8.0 accept · 10.0.0-alpha accept · 10.0.0-alpha.1 accept
  abc reject · 1.2 reject