fix(renderer): rescue failed Linux Auto presentation per launch

Probe Qt's default backend in a disposable graphics-only child before runtime and the main window. After proven failure, allow one software presentation attempt without persisting a renderer preference. Preserve explicit backend policy and saved-GPU validation.

Bound and reap children, keep deep links in the parent, and avoid blocked diagnostic pipes. Cover healthy/failing decisions, overrides, missing executables, paths with spaces and watchdog cleanup in the passing Qt suite.

cx

crates/qbz-qt/Cargo.toml

@@ -125,6 +125,7 @@ qbz-player = { path = "../qbz-player" }
 # (StreamExt over the response byte stream).
 uuid = { version = "1", features = ["v4"] }
 futures-util = { workspace = true }
+tempfile = "3"
 async-trait = "0.1"
 zeroize = "1"