Use fork-sync and make a local copy of fmt

[vmoroz]

Description

Type of Change

• New feature (non-breaking change which adds functionality)

Why

Third-party dependencies in RNW (fmt, Folly, ReactCommon) use a bespoke
MSBuild download-unzip-overlay pipeline that has no conflict detection, no
3-way merge, and duplicates the same concepts per library. This PR begins
adopting @rnx-kit/fork-sync — the same tool used in hermes-windows — to get
Git 3-way merges, AI-assisted conflict resolution, and a unified
manifest-driven workflow.

fmt is the first dependency to migrate because it is the simplest case: a
small library with no active patches.

What

Phase 0 — fork-sync infrastructure:

• Created @rnw-scripts/fork-sync package — thin wrapper that invokes
  @rnx-kit/fork-sync with -C pointing to the repo root.
• Added sync-manifest.json at the repo root listing fmt as the first
  managed dependency.
• Added /.sync/ to .gitignore (fork-sync's working directory).
• Added fork-sync and fork-sync:status scripts to root package.json.

Phase 1 — migrate fmt to fork-sync:

• Committed fmt 11.0.2 source into vnext/external/fmt/ with
  sync-config.json, .syncignore, and cgmanifest.json.
• Added vnext/external/.clang-format (DisableFormat: true) to prevent
  formatting of vendored source files.
• Moved fmt ClCompile/ClInclude entries into Shared.vcxitems — fmt is
  now compiled inline instead of as a separate static library.
• Added a shared <ClCompile Update="$(ExternalDir)**"> block in
  Shared.vcxitems so all future external sources automatically get the
  right compiler settings (no PCH, no WinRT, common preprocessor defs).
• Defined $(ExternalDir) in Directory.Build.props pointing to
  vnext\external\.
• Updated fmt include paths in React.Cpp.props, Folly.vcxproj, and
  Microsoft.ReactNative.vcxproj from $(FmtDir)include to
  $(ExternalDir)fmt\include.
• Removed FmtVersion, FmtCommitHash, and FmtDir properties from
  Directory.Build.props.
• Deleted vnext/fmt/ directory (vcxproj, filters, pch files,
  clang-format, packages.lock.json).
• Removed fmt project entry and build configurations from 12 .sln files
  and 2 .slnf files.
• Removed ProjectReference to fmt from Folly.vcxproj.

Screenshots

N/A

Testing

• Built Microsoft.ReactNative.vcxproj (Debug|x64) — fmt compiles
  correctly as part of Shared.
• Built Folly.vcxproj (Debug|x64) — finds fmt headers via updated
  include path.
• Built React.Windows.Desktop.DLL.vcxproj (Debug|x64) — Desktop target
  also works.
• Verified yarn format no longer reformats vendored fmt source (thanks
  to vnext/external/.clang-format).

Changelog

Should this change be included in the release notes: no

Microsoft Reviewers: Open in CodeFlow

[Saadnajmi]

Can we not pull fmt as a package and patch it if need be? Do we need a source copy?

[vmoroz]

Can we not pull fmt as a package and patch it if need be? Do we need a source copy?

All major OSS projects typically have a local copy of 3rd party code. It makes it easy to support them.
For us having the sources in our repo makes it easy to investigate issues in the crash dumps and do any quick changes.
The new fork-sync tool makes it trivial to support such local forks.

[Saadnajmi]

Can we not pull fmt as a package and patch it if need be? Do we need a source copy?

All major OSS projects typically have a local copy of 3rd party code. It makes it easy to support them. For us having the sources in our repo makes it easy to investigate issues in the crash dumps and do any quick changes. The new fork-sync tool makes it trivial to support such local forks.

That's not true? Most OSS projects use a package or dep manager to download them?