[build-tools] Add local Xcode compilation cache

[sjchmiela]

Why

Xcode 26 has a compilation cache, but C and Objective-C cache tasks require a remote service path. EAS needs a local-only cache that can be archived after a build and restored on another CI machine. This must not require Bazel, Tuist, or a remote CAS service.

The deprecated worker CacheManager is not used for this feature. The implementation uses the active build-cache path that already restores and saves ccache and the Gradle build cache.

How

• Add a local CAS shim that loads Apple's Xcode CAS plugin.
• Forward every llcas_* operation to Apple's implementation.
• Accept and discard only the remote-service-path option. This makes Xcode schedule C, Objective-C, PCH, and PCM cache tasks without remote I/O.
• Enable the feature explicitly with EAS_BUILD_XCODE_COMPILATION_CACHE=1.
• Build one arm64 shim during iOS worker packaging and store it next to record-sim.
• Load Apple's CAS plugin from the selected Xcode at build time, so the bundled shim does not bind to one Xcode installation.
• Validate the bundled shim with Xcode's llvm-cas before enabling the cache. Disable compilation caching when the shim is missing or incompatible.
• Set GYM_DERIVED_DATA_PATH so Fastlane and cache restore/save use the same ios/build directory.
• Store the cache in ios/build/CompilationCache.noindex.
• Restore and save it through restoreBuildCache.ts and saveBuildCache.ts, next to ccache and Gradle cache handling.
• Run ccache and Xcode cache restore/save operations in parallel.
• Use a separate cache key that includes the exact Xcode version and dependency lock hash.
• Keep sparse-file archive and restore handling inside the Xcode compilation-cache module. The generic cache helpers are unchanged.
• Support both the traditional iOS builder and the EAS build step path.

Test Plan

• Build the C shim as arm64 with -Wall -Wextra -Werror and confirm it links only to libSystem.
• Validate the shim with llvm-cas and ingest a file into a temporary CAS.
• Build a temporary Objective-C Xcode project twice and confirm that the second clean build replays cache hits.
• Archive, remove, restore, and reuse CompilationCache.noindex in another clean build.
• Archive and restore a real 122 MB Xcode CAS directory:
  • archive: about 1 second
  • restore: about 1 second
  • all 35 files restored
• Benchmark seven cold and seven warm clean builds:
  • cold median: 1.821 seconds
  • warm median: 0.955 seconds
  • about 48% faster for the test project
• Run all build-tools unit tests:
  • 107 suites passed
  • 1,018 tests passed
• Run build-tools type checking.
• Validate the worker packaging shell script.
• Run repository lint and format checks. Lint completes with 20 existing unrelated warnings and zero errors.

[codecov]

Codecov Report

❌ Patch coverage is 76.63551% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.56%. Comparing base (3cc58e5) to head (8a04ae6).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/build-tools/src/ios/compilationCache.ts	84.49%	9 Missing ⚠️
...ild-tools/src/steps/functions/restoreBuildCache.ts	65.22%	8 Missing ⚠️
.../build-tools/src/steps/functions/saveBuildCache.ts	75.00%	5 Missing ⚠️
...ckages/build-tools/src/steps/utils/ios/fastlane.ts	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4204      +/-   ##
==========================================
+ Coverage   63.53%   63.56%   +0.03%     
==========================================
  Files        1028     1029       +1     
  Lines       46905    47019     +114     
  Branches     9842     9862      +20     
==========================================
+ Hits        29798    29883      +85     
- Misses      17005    17034      +29     
  Partials      102      102              

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[github-actions]

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