[Bug] Native UI verifier can lose fixture ownership before the ready handshake #8559

Area

Native UI verifier cleanup

Observed behavior

At revision 245a581818ab13694fa438a06e39898ce03ab206, verify-native-ui.mjs records a fixture PID only after the window-ready output is parsed. If the handshake times out, cleanup signals and waits for the open launcher, then removes the fixture root without an identity for a separately launched fixture process. Launch and handshake; cleanup.

Reproduction and evidence

On macOS arm64, Node 24.22.1, a subprocess ran the actual verifier with injected compiler and open launcher, and an accelerated clock. The simulated launcher started a detached, source-owned Node fixture that never sent a ready record. After the verifier timed out and terminated its launcher, the fixture was still present as a sleeping process reparented to PID 1; it was then explicitly terminated by the test operator. An independent source review confirmed that this follows the verifier's missing-handshake cleanup path.

This simulates a separate fixture process. Actual macOS LaunchServices behavior, real GUI survival, and incidence are NOT_ENOUGH_INFO; no native compiler, GUI, or provider was run.

Expected behavior

Establish and retain ownership of the launched fixture through late or missing handshake and cleanup; do not delete the fixture root while an owned app may remain. Any fix should use process-ownership primitives and cover late handshake, startup failure, cleanup refusal and retry, without signaling a process from PID or path alone.