Add temporary launcher icon

[R055LE]

Summary

• replace the blank-looking launcher glyph with the owner-supplied temporary artwork
• provide density-specific legacy icons plus an adaptive icon and round-icon manifest entry
• retain the monochrome terminal vector for the foreground-service notification
• cover the packaged launcher resource with an API 35 instrumentation regression

Validation

• ./gradlew test lint assembleDebug
• ./gradlew :app:connectedDebugAndroidTest -Pandroid.testInstrumentationRunnerArguments.class=dev.threadline.LauncherIconTest
• API 35 Pixel launcher and app-switcher visual checks
• exactly one io.github.r055le.threadline.debug launcher activity resolved

No version bump or dedicated signed alpha. Owner-device update and launcher-mask acceptance remain for the next shared candidate.

Closes #38