Update Windows x64 to CEF 6613

[summeroff]

Folded into #775. The CEF 6613 commits were rebased with the sandbox stack onto current streamlabs and are now reviewed and built from #775. This stacked PR is closed with no remaining diff.

Summary

• update only the Windows x64 CEF dependency from 6533 v4 to 6613
• add a backward-compatible per-platform dependency version override so macOS, Linux, and Windows ARM64 remain on their existing CEF packages
• teach Windows symbol validation to use the selected platform-specific CEF version
• generate a GUID/age-matched public libcef PDB in the symbol-upload staging tree, while retaining private symbols in the downloadable artifact
• add a reproducible link-only helper that converts Chromium's 8 KiB PDB layout to 4 KiB before public-symbol extraction
• update the legacy Windows cache key to 6613

Artifact

CEF 6613 revision 2 is built and locally verified, but is not yet uploaded to the public CEF bucket:

• cef_binary_6613_windows_x64_v2.zip
• size: 875,213,740 bytes
• SHA-256: C8FA34997506FC09712EDDDA65E029A3BCC8B673B45E12EEE7A091142F61A8E2
• companion symbols: 403,431,623 bytes, SHA-256 A93F4C3B29149971C7BFC3E069D14E9A97F0C25B739666A3B68E6EB5C204624E

The preset intentionally remains on revision 1 until the v2 runtime archive is uploaded and its public URL returns HTTP 200. The final revision-2 hash above must then replace the revision-1 value before this PR is marked ready.

Dependency

• stacked on Enable the Windows CEF sandbox and update x64 to CEF 6613 #775; retarget to streamlabs after that PR merges
• no additional obs-browser source changes are required for 6613

Symbol strategy

Chromium 128's bundled lld-link advertises /PDBSTRIPPED but rejects it as unimplemented. Its default 8 KiB-page PDB also produces EC_FORMAT with the installed pdbcopy 14.00.23615. Relink-CefWithCompatiblePdb.ps1 safely repeats only the final link from Ninja's exact response file with 4 KiB PDB pages, without changing the source build outputs. Create-CefPublicPdb.ps1 then uses the supported Windows SDK pdbcopy -p flow, verifies GUID/age identity and stripped state with llvm-pdbutil, enforces the CAB size ceiling, and replaces only the symbol-server job's temporary copy. The original private PDB remains in both downloadable artifacts.

Validation

• Release-only CEF 6613 build with symbol_level=1; no Debug payload
• final runtime archive is root-flat and has the same 1,097 paths as revision 1; only Release/libcef.dll and its PDB changed
• both runtime and symbols archives pass 7z t; all eight PDB paths are retained and byte-identical between archives
• relinked DLL/private PDB match: GUID {345868A3-286C-82CA-4C4C-44205044422E}, age 1, 4 KiB blocks
• exact 2,204,463,104-byte private PDB converts to a matching stripped 383,627,264-byte public PDB
• the public PDB compresses to one valid 55,858,032-byte CAB
• end-to-end symstore.exe add /compress accepts it and creates the expected GUID/age-keyed 55,858,025-byte single-file store entry
• libcef import and export sets are unchanged by the link-only page-size conversion
• revision 1 configured and built successfully with CEF 128.0.0, including Debug and RelWithDebInfo browser/sandbox targets and tests

Remaining gates

• upload revision 2 and verify the public URL/hash before updating CMakePresets.json
• run the tag release workflow to exercise the complete public-PDB symbol upload
• retain draft status until the complete sandbox stack has real restricted-token and job-object evidence