[ashmitvoid]

Summary

• remove the invalid lineHeight assignment from the TextEdit in GitDiffPanel.qml
• add a headless ApplicationStartupSmoke.sh that instantiates the real root QML under QT_QPA_PLATFORM=offscreen
• fail CI on root-component load failure, unavailable QML dependency types, non-existent property assignments, or unexpected early application exit
• run the smoke inside a fresh D-Bus session immediately after the normal unit suite

Target-machine failure fixed

Exact candidate 8f82b5c5ccfbea03babb67244b3e42baa43d4ad4 passed 29/29 main tests and 4/4 preview tests but failed at real launch with:

QQmlApplicationEngine failed to load component
qrc:/qt/qml/Ryofiles/qml/Main.qml:860:9: Type FileContextMenu unavailable
qrc:/qt/qml/Ryofiles/qml/components/FileContextMenu.qml:727:5: Type GitDiffPanel unavailable
qrc:/qt/qml/Ryofiles/qml/components/GitDiffPanel.qml:272:25: Cannot assign to non-existent property "lineHeight"

The root cause is the TextEdit.lineHeight assignment in GitDiffPanel.qml. Removing it preserves the bounded diff viewer and allows the component chain to instantiate on the target Qt runtime.

Validation policy

Frozen head: 868f3e6b6d99442bc5b6bc265da03ff4140da8a5
Base: 8f82b5c5ccfbea03babb67244b3e42baa43d4ad4

Merge only if Build and Packaging are green on this exact head. After merge, target-machine validation must restart from the new main SHA; evidence from 8f82b5c5… is superseded.

Closes #90.
Blocks #72 and #69 until target-machine retest passes.