Extend windows-latest + macos-latest post-merge triggers to release/v* #293

Sibling of milestone #13. Surfaced during #291's review (which extended the PR-trigger workflows to release/v* but deliberately left push triggers alone).

Problem

After #291 + #292, PRs targeting release/v* get ubuntu-latest CI validation. But the post-merge cross-platform validation workflows — windows-latest.yml and macos-latest.yml — still only fire on OnPushBranches: [main]. So release/v11 doesn't get the same post-merge Windows + macOS validation that main does after every merge.

If a hotfix lands on release/v11 with a subtle Windows or macOS regression, we wouldn't catch it until somebody attempts a release and the cross-platform CI in release.yml's Test step fails — too late, and on a higher-stakes path.

Fix

Extend the OnPushBranches attribute on the windows-latest and macos-latest [GitHubActions] entries in build/Build.CI.GitHubActions.cs to include ReleaseBranchPattern alongside MainBranch. Same shape as OnPullRequestBranches post-#291.

Re-regenerate the two YAML files (dotnet fallout --generate-configuration GitHubActions_windows-latest GitHubActions_macos-latest --host GitHubActions) or hand-edit them to match.

Acceptance criteria

• build/Build.CI.GitHubActions.cs — windows-latest and macos-latest OnPushBranches arrays include both MainBranch and ReleaseBranchPattern.
• .github/workflows/windows-latest.yml and .github/workflows/macos-latest.yml regenerated (or hand-edited) to match.
• Verified by a smoke push to release/v11 (a CHANGELOG bump or similar low-risk change) triggering the cross-platform workflows.

Why not in #291

#291 was the chicken-and-egg unblock for PR-targeting release/v*. Extending push triggers is a parallel concern (post-merge validation) with no dependency chain, and bundling it into #291 would have widened scope at a time-sensitive moment.