Automate post-release baseline propagation and gate on nuget indexing

[Vulthil]

Summary

• Gate release.yml's baseline-bump PR on nuget.org having indexed the released version, so its restore doesn't race indexing and fail with NU1102.
• Add a propagate-baseline-to-main job that, after a servicing-branch release, bumps main's PackageValidationBaselineVersion to match — upward only, and skipped entirely when the release is dispatched from main itself.
• Catch up main's baseline to 1.0.2 now, since the last servicing release never propagated here.

Verification

• Ran eng/propagate_baseline.py against scratch Directory.Build.props copies for all four scenarios (newer/older/equal/missing-property).
• actionlint on the changed workflow: no new findings.
• dotnet build -c Release succeeds against the 1.0.2 baseline.

Backport to v1.0: yes — release workflows execute per-branch; companion PR #308

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.