Compile auto-generated Swift sources (Bundle.module, asset symbols) in Tier 2

[obj-p]

Summary

Three fixes around SPM/Xcode build-system integration:

1. SPM: walk <binPath>/<Target>.build/DerivedSources/*.swift and append to the compile set. Surfaces resource_bundle_accessor.swift so previews using Bundle.module build.
2. Xcode: walk <DERIVED_FILE_DIR>/DerivedSources/*.swift and append. Surfaces GeneratedAssetSymbols.swift, GeneratedStringSymbols.swift, etc. so previews using Color(.brandPrimary) / Image.foo build.
3. SPM build-state recovery: auto-retry swift build after cleaning .build/<triple>/ + .build/build.db when llbuild reports "command X not registered" (a known incremental-state bug across cross-process / cross-triple invocations).

Bazel is documented-N/A — rules_swift.swift_library does not synthesize a Bundle.module equivalent. Comment records known adjacent gaps (swift_proto_library, swift_grpc_library, consumer-macro outputs).

The first two are pure static helpers plus a single-line call site in build(). No change to BuildContext or Compiler — the existing sourceFiles: [URL]? → Compiler.compileCombined(additionalSourceFiles:) pipeline already carries arbitrary URLs.

Failing-before repros

// examples/spm/Sources/ToDo/ToDoView.swift
value: items.first { !$0.isComplete }?.title
    ?? Bundle.module.localizedString(forKey: "todo.empty.title", value: "All done!", table: nil),

$ previewsmcp snapshot examples/spm/Sources/ToDo/ToDoView.swift -o /tmp/out.png --project examples/spm
# Before: error: type 'Bundle' has no member 'module'
# After: /tmp/out.png

// examples/xcodeproj/Sources/ToDo/ToDoView.swift
color: Color(.brandPrimary)   // symbol from Assets.xcassets/BrandPrimary.colorset

$ (cd examples/xcodeproj && xcodegen generate) \
  && previewsmcp snapshot examples/xcodeproj/Sources/ToDo/ToDoView.swift -o /tmp/out.png --project examples/xcodeproj
# Before: error: type 'Color' has no member 'brandPrimary'
# After: /tmp/out.png

# Cross-invocation flake (reproduces on main; fixed by SPMBuildRecovery)
$ rm -rf examples/spm/.build examples/PreviewSetup/.build && previewsmcp kill-daemon
$ swift test --filter PreviewsCoreTests        # 0 failures
$ swift test --filter CLIIntegrationTests
# Before: 99 failures cascading from "command X not registered"
# After:  0 failures

Example updates

• examples/spm/ declares resources: [.process("Resources")] and ships Localizable.xcstrings; ToDoView reads Bundle.module.localizedString(...).
• examples/xcodeproj/ and examples/xcworkspace/ ship Assets.xcassets/BrandPrimary.colorset, set ASSETCATALOG_COMPILER_GENERATE_SWIFT_ASSET_SYMBOL_EXTENSIONS: YES explicitly (the library-target default isn't reliable across XcodeGen versions), and ToDoView uses Color(.brandPrimary).
• XcodeGen version used locally: 2.44.1. pbxproj is gitignored per repo convention; CI regenerates via xcodegen generate (the existing snapshotXcodeproj test already does this).

Regression coverage

Existing integration tests now transparently exercise the generated-source paths because the examples they drive use Bundle.module / Color(.brandPrimary):

• SnapshotCommandTests.basicMacOSSnapshot (SPM)
• SnapshotCommandTests.snapshotXcodeproj (Xcode project)
• SnapshotCommandTests.snapshotXcworkspace (Xcode workspace)

New unit tests for the helpers (no swift build / xcodebuild required):

• BuildSystemTests.collectGeneratedSources_spm_findsResourceAccessor / _emptyWhenMissing
• BuildSystemTests.collectGeneratedSources_xcode_findsDerivedSwift / _emptyWhenMissing
• SPMBuildRecoveryTests.parseStaleTripleDirectory* (4 cases)

Test plan

• swift test --filter 'BuildSystemTests' — 57 tests pass
• swift test --filter 'SPMBuildRecoveryTests' — 4 tests pass
• swift test --filter 'SnapshotCommandTests.basicMacOSSnapshot' passes after fix
• swift test --filter 'SnapshotCommandTests.snapshotXcodeproj' passes
• swift test --filter 'SnapshotCommandTests.snapshotXcworkspace' passes
• swift test --filter PreviewsCoreTests && swift test --filter CLIIntegrationTests — 0 failures (was 99 before SPMBuildRecovery)
• Manual: previewsmcp snapshot against both examples produces valid PNGs

Known adjacent issues (not fixed here)

• The thunk generator transforms string literals into DesignTimeStore.shared.string(...) returning String, which breaks String(localized: "...", bundle: .module) (requires String.LocalizationValue). The example uses Bundle.module.localizedString(forKey:value:table:) instead. Fixing the thunk generator is out of scope for this PR.

🤖 Generated with Claude Code