ci: stable Windows required summary for protected main (#61)

[ShutenOishi]

Issue #61: fixed-name required Windows CI (no release changes)

Pilot source: pgextwin/plpgsql_check PR #26 (merged after all PG14–18 Windows jobs and stable summary succeeded).

• Add Windows required summary, dependent on the entire windows reusable Windows matrix, not individual PG14/PG15/... check names.
• Gate runs with always() and accepts only literal needs.windows.result == success. Failed, cancelled, skipped and missing results deny. Existing full-SHA-pinned resolve-matrix.ps1 throws if zero maintained PG majors, so empty matrix cannot yield successful reusable workflow.
• Add standalone fail-closed positive/negative fixture tests, run by this summary job.
• Current manual PUBLISH-only Release workflow, candidate dispatch/report, signed assets, branch settings and all existing releases remain unchanged.

Do not set this as required branch protection until its exact-head PR CI and main push CI both succeed. No independent reviewer requirement. After CI acceptance, Work may configure Windows required summary as the stable required check, preserving all other required checks. Tracking pgextwin/build#61.