ci: add RC prerelease and release-branch workflows

[spbsoluble]

Summary

• Adds pr-prerelease.yml: on each push to a PR targeting a release-X.Y branch, automatically computes and pushes a vX.Y.Z-rc.N tag (0-based, increments per push) and publishes a GitHub prerelease + NuGet package
• Rewrites release.yml: triggers on push to release-* branches (i.e. on merge) instead of push to v* tags — necessary because tags created by GITHUB_TOKEN inside a workflow do not re-trigger other workflows
• On merge to release-X.Y, creates vX.Y.Z release tag where Z = latest non-RC patch + 1
• Adds scripts/test-semver.sh: standalone bash script that tests the version computation logic from both workflows without requiring a git repo, network access, or any commits

Version logic

Scenario	Example
Latest tag is v1.0.0, PR opened into release-1.0	v1.0.1-rc.0
Second push to same PR	v1.0.1-rc.1
PR merged	v1.0.1
Next PR opened	v1.0.2-rc.0

Test coverage (scripts/test-semver.sh)

• RC and release versioning isolated by exact X.Y — tags from other majors/minors are invisible
• Zero-based RC counter increments correctly across multiple pushes
• RC tags are ignored when computing the next release Z
• sort -V handles double-digit patch numbers correctly (9 → 10, not lexicographic)
• Regression case: PR into release-1.2 with v2.0.x tags present produces v1.2.x-rc.0, not a v2.x tag
• End-to-end stateful lifecycle: PR open → pushes → merge → next PR

Test plan

• Open a PR targeting a release-X.Y branch and confirm vX.Y.Z-rc.0 tag and prerelease are created
• Push a second commit to the PR and confirm vX.Y.Z-rc.1 is created
• Merge the PR and confirm vX.Y.Z release tag and full release are created
• Confirm NuGet package is published to GitHub Packages for both RC and release
• Run bash scripts/test-semver.sh and confirm all 23 tests pass