SwiftPM: stop baking an absolute HERMES_CLI_PATH into the app pbxproj

[chrfalch]

Summary:

npx react-native spm add writes a HERMES_CLI_PATH build setting into the app's project.pbxproj. The value is the absolute path of hermesc inside the hermes-compiler npm package, resolved on the machine that ran the command, for example /Users/<me>/repos/<app>/node_modules/hermes-compiler/hermesc/osx-bin/hermesc. The pbxproj is committed, so every SwiftPM-converted app commits one developer's disk layout to its repository.

The setting is redundant. react-native-xcode.sh already resolves hermesc at build time through react-native's own dependency graph when the current HERMES_CLI_PATH is not a file, using the same hermes-compiler version the artifact download pins (#57928). A relative build setting is not an option: $(REACT_NATIVE_PATH)/../hermes-compiler breaks when react-native is a symlink, and $(SRCROOT)/../node_modules/... breaks for hoisted monorepos.

This change:

• Removes the HERMES_CLI_PATH write from generate-spm-xcodeproj.js. Existing injected projects clean themselves up on the next spm add or spm update, because removeRecordedBuildSettings already strips every scalar recorded in .spm-injected.json before re-injecting.
• Fixes the shell fallback's gate. It was keyed on PODS_ROOT being absent, so a SwiftPM app that keeps side-by-side non-RN pods never reached it and had no hermesc. It is now keyed on the hermes-engine pod directory being absent, with Hermes enabled, so every app that has the pod keeps its previous behavior exactly, including a partial prebuild still failing the build.

Changelog:

[IOS] [FIXED] - SwiftPM: stop baking an absolute, machine-specific HERMES_CLI_PATH into the app's pbxproj; resolve hermesc at build time instead

Test Plan:

Red first. New test at the entry point that resolved the path, with a hermes-compiler fixture seeded so the old code provably wrote the setting:

yarn jest packages/react-native/scripts/spm/__tests__/remove-spm-injection-test.js --no-cache -i
# with the source changes stashed:
● injectSpmIntoExistingXcodeproj — HERMES_CLI_PATH › writes no HERMES_CLI_PATH, in any configuration or the marker
    Expected substring: not "HERMES_CLI_PATH"
    HERMES_CLI_PATH = "/private/var/.../node_modules/hermes-compiler/hermesc/osx-bin/hermesc";
    HERMES_CLI_PATH = "/private/var/.../node_modules/hermes-compiler/hermesc/osx-bin/hermesc";
Tests: 1 failed, 76 skipped, 77 total

Green after the change:

yarn jest packages/react-native/scripts/spm --no-cache -i
Test Suites: 18 passed, 18 total
Tests:       834 passed, 834 total

Also: yarn eslint --max-warnings 0 and yarn prettier --check on the changed files, yarn flow-check (0 errors), bash -n packages/react-native/scripts/react-native-xcode.sh.

End to end: a React Native 0.87.1 SwiftPM app (@shopify/react-native-skia example, no Pods) with the HERMES_CLI_PATH lines deleted from its pbxproj builds in Release. The bundling phase logs the fallback resolving hermesc from node_modules/hermes-compiler, and main.jsbundle is produced. There is no shell test harness for react-native-xcode.sh, so that path is covered by this build only.

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