Enable sccache on Windows runners

[crohr]

Supersedes #32. Closes #31.

The sccache input was ignored outside Linux, so Windows runners never received the RunsOn S3 cache configuration.

This change:

• enables sccache on RunsOn Linux and Windows runners;
• adds a Windows 2025 workflow that requires cache hits on the second build;
• keeps the current bootstrap runner labels and action versions;
• updates the public platform contract; and
• rebuilds all distributed binaries from the resolved source.

The source commit credits Adam Gutglick for the original contribution.

Validation:

• go test ./...
• go test -race ./...
• actionlint .github/workflows/test-sccache.yml
• UPX integrity checks for all three distributed binaries

[crohr]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Delightful!

Reviewed commit: 27db62dde5

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