ci: overlap Windows media setup

[SunkenInTime]

What changed

• start Media Foundation preparation before checkout as an independent Windows scheduled task
• overlap the 37-58.8 second install with checkout, npm, Zig setup, and native cache restoration
• wait and verify immediately before native compilation, with actionable failure output
• audit the ordering so the overlap cannot regress silently

Benchmark

• previous warm master: Windows native 2:25, full workflow 2:59
• final PR proof: Windows native 1:40, full workflow 2:31
• Media Foundation: 58.8 seconds total, 22 seconds visible wait
• exact Zig build cache hit on AMD gen-2

The PR workflow still included 50 seconds of Blacksmith Windows queue time, separate from the 1:40 job runtime.

Verification

• full CI green: https://github.com/unmde/weaver/actions/runs/32758322275
• npm run audit:release
• git diff --check
• workflow YAML parse