Stabilize Windows modern compatibility package by auto-cloning missing codec sources

[Copilot]

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Summary

• Linked issue or task: GitHub Actions failure in Windows modern compatibility package (job 80250207764)
• Description of changes: The Windows nightly lane failed on clean runners because src/libs/_deps was missing and codec bootstrap required pre-populated sources. This change makes bootstrap self-healing by cloning missing codec trees before configure/build.
  • Root cause
    • tools/build_internal_deps.ps1 failed fast when libogg/libvorbis/zlib/libpng source trees were absent under src/libs/_deps.
  • Bootstrap fix
    • Replaced strict source-presence checks with Ensure-GitDependencySource for:
      • libogg (xiph/ogg, v1.3.5)
      • libvorbis (xiph/vorbis, v1.3.7)
      • zlib (madler/zlib, v1.3.1)
      • libpng (glennrp/libpng, v1.6.43)
  • Regression guard
    • Extended tests/test_renderer_font_build_lane_parity.py to assert these git-backed bootstrap calls exist.
  • Docs alignment
    • Updated BUILD.md and docs/build/windows.md to reflect automatic _deps source hydration via git + cmake.

Ensure-GitDependencySource -Name 'libogg' `
  -SourceDir $oggSourceDir `
  -RepositoryUrl 'https://github.com/xiph/ogg.git' `
  -Tag 'v1.3.5' `
  -RequiredPaths @(...)

Verification

• I have run the relevant harness suites locally and attached the results below.
• I have updated the project baselines when required and linked the diff below.
• I have documented any syscall contract changes and linked the record below.

Required Links (fill every field or provide a justified N/A)

• Harness run(s): N/A – issue scope is Windows build bootstrap in CI, not runtime harness behavior.
• Baseline update diff: N/A – no gameplay/runtime baseline behavior changed.
• Syscall contract edit log: N/A – no syscall boundaries or contracts changed.

Additional Notes

• Expected impact: Windows modern compatibility package can now run from a fresh checkout without manual pre-population of src/libs/_deps.