test(ios): automated Swift DCE verification for `#if !DISABLE_*` guards

[kirich1409]

Goal

Add an automated test that proves the Swift compiler eliminates code guarded by #if !DISABLE_* from release binaries, mirroring featured-shrinker-tests for Android R8.

Context

The Gradle plugin generates FeatureFlags.generated.xcconfig with SWIFT_ACTIVE_COMPILATION_CONDITIONS = $(inherited) DISABLE_<FLAG_KEY> for each local flag whose defaultValue = false:

• Generator: build-logic/featured-gradle-plugin/src/main/kotlin/dev/androidbroadcast/featured/gradle/XcconfigGenerator.kt
• Sample usage: iosApp/iosApp/ContentView.swift (#if !DISABLE_* guards)
• Closed shipping work: feat(gradle-plugin): generate FeatureFlags.generated.xcconfig for iOS Swift DCE #18 (xcconfig generator), feat(ios): document and enforce #if entry point pattern for Swift DCE #24 (Swift #if pattern doc).

Today there is no automated verification that Swift DCE actually strips the guarded branches from the release binary — the equivalent of featured-shrinker-tests/src/test/kotlin/dev/androidbroadcast/featured/shrinker/R8BooleanFlagEliminationTest.kt is missing for iOS.

Approach

• Add a featured-swift-dce-tests Gradle module (skipped on non-macOS hosts).
• Generate xcconfig via the existing plugin task.
• Compile a small Swift fixture (swiftc -O -D DISABLE_X) — bypass full Xcode to keep the test fast.
• Read symbols from the resulting object/binary (nm or swift demangle) and assert that symbols from the disabled branch are absent.
• Verify the opposite case (no -D DISABLE_X) keeps the symbol present.

Definition of Done

• New test module wiring xcconfig generation
• Test compiles a Swift fixture with -O and asserts symbol stripping
• CI job runs the test on the macOS runner (existing iOS build job already runs there)
• Wiki page iOS — xcconfig DCE references the test
• CHANGELOG.md [Unreleased] entry

Related

• feat(gradle-plugin): generate FeatureFlags.generated.xcconfig for iOS Swift DCE #18, feat(ios): document and enforce #if entry point pattern for Swift DCE #24 (closed), test(gradle-plugin): integration tests for ProGuard rule generation and R8 DCE #28 (Android R8 integration), Automated R8 DCE verification for feature flags #153 (Android R8 coverage).