test(🍏): minimal SwiftPM example app and CI job

[chrfalch]

Motivation

Stacked on #4043, which adds packages/skia/Package.swift. Nothing exercises that manifest without this PR. It pins a source list, four header search paths, six defines and two relative package paths that hold only because autolinking uses a symlink. Any of those can break on a React Native bump, silently.

Base is main because a pull request's base must be a branch in the base repository, and this one comes from a fork. The first two commits belong to #4043; this PR's own change is the third — review it alone via 5f6ae3d. The full diff reduces once #4043 lands. Push spm-support to this repository and I will retarget the base for real stacked review.

What is here

spm-example/: a bare React Native 0.87.1 app, no CocoaPods, drawing a Skia canvas, plus the build-test-ios-spm job that installs and builds it.

It sits outside the yarn workspace on purpose: SwiftPM needs React Native 0.87 or newer, apps/example is on 0.83.1, and the two resolutions must not collide. It is therefore not a turbo workspace, so root yarn lint and yarn test do not reach it.

Four deliberate oddities:

• No lockfile. react and react-native are pinned exactly, so the native build stays deterministic and CI installs with npm install. This keeps a 12,000-line generated file out of review.
• ios/Podfile is not for CocoaPods. React Native's CLI locates the iOS project by finding a Podfile and taking its directory as sourceDir; with none it returns null, and both react-native spm and the Xcode autolinking phase fail with CLI config did not provide project.ios.sourceDir. userConfig.sourceDir only seeds that search, so config cannot override it. The file installs nothing and calls raise, so a stray pod install fails clearly.
• CI runs spm update, not spm add --deintegrate. The .xcodeproj is committed already injected, and an injected project resolves to update, which also keeps CocoaPods off the runner.
• No HERMES_CLI_PATH committed. React Native 0.87.1's injector writes an absolute path to hermesc into the project. The build does not need it, since react-native-xcode.sh resolves hermesc itself, and committing it would put one developer's disk layout in the repository. The README documents the strip.

Test plan

The exact CI sequence, from a clean install:

rm -rf node_modules && npm install     # no lockfile → react-native 0.87.1, react 19.2.3
cd ios && npx react-native spm update  # exit 0, "Self-managed: ReactNativeSkia"
xcodebuild -project SpmExample.xcodeproj -scheme SpmExample \
  -configuration Debug -sdk iphonesimulator \
  -destination 'generic/platform=iOS Simulator' CODE_SIGNING_ALLOWED=NO build

• Debug: ** BUILD SUCCEEDED **. No CocoaPods invoked, and the Podfile's raise never fires.
• Release builds, bundles and renders on the simulator. The bundling phase resolves hermesc through the fallback, confirming the omitted setting is unnecessary.
• Red: with Package.swift absent the setup exits 2 — the failure this harness exists to catch.
• The tree stays clean after a full run, and no committed file contains an absolute path.

build-test-ios-spm has never run anywhere before, so this PR is its first run.