feat(host): isolate the zero-copy GPU import in a worker process

The tiled EGL/GL→CUDA import crashed the whole host (SIGSEGV inside
libnvidia-eglcore via cuGraphicsMapResources) when the compositor
invalidated an imported dmabuf mid-map — reproduced on the Bazzite F44
Game→Desktop switch (design/zerocopy-hardening-handoff.md). A driver
SIGSEGV is uncatchable in-process, so the whole EglImporter (tiled
EGL/GL→CUDA and LINEAR Vulkan→CUDA) now runs in a per-capture
`zerocopy-worker` subprocess: dmabuf fds go over a SEQPACKET socketpair
(SCM_RIGHTS, sent once per buffer keyed by dmabuf st_ino; NeedFd resend
self-heals cache desync), frames come back as CUDA-IPC pooled device
buffers (still zero-copy, +one socket RTT/frame). Worker death poisons
the capturer so the existing capture-loss rebuild runs — the host
survives; 3 consecutive deaths latch the GPU import off (CPU/SHM path).
PUNKTFUNK_ZEROCOPY_INPROC=1 keeps the old in-process import for
debugging/A-B.

Also fixed along the way: a failed *tiled* import no longer falls
through to the CPU mmap de-pad (which scrambled tiled bytes; LINEAR
keeps the fallback); Nv12Blit dropped its GL textures while still
CUDA-registered (unregister now runs first); GlBlit had no Drop at all
(GL objects leaked per size change); VkBridge's per-fd src cache is now
invalidated on renegotiation/eviction instead of never.

Design: design/zerocopy-worker-isolation.md. Unit tests: 14 new
(protocol fd-passing, worker dispatch, client handshake/death/NeedFd,
death latch). On-glass validated on the RTX 5070 Ti/GNOME box (.21):
the worker path streams at p50 1.30 ms (NV12, 1800 frames 0-mismatched,
parity with the in-process path), and a kill -9 of the worker
mid-stream is survived by the host and recovered — poison -> capture
lost, rebuilding pipeline in place -> a fresh worker in ~185 ms ->
streaming resumes (2385 frames, 0 mismatched). A real KWin
compositor-crash repro is still pending (a worker kill -9 is strictly
harsher, so it corroborates).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/punktfunk-host/src/main.rs

@@ -181,6 +181,11 @@ fn real_main() -> Result<()> {
         // Zero-copy FFI/GPU probe: init the EGL importer + CUDA context (no capture needed).
         #[cfg(target_os = "linux")]
         Some("zerocopy-probe") => zerocopy::probe(),
+        // Hidden: the isolated GPU-import worker the capture path spawns from /proc/self/exe
+        // (design/zerocopy-worker-isolation.md) — never run by hand; --fd names the inherited
+        // socketpair end.
+        #[cfg(target_os = "linux")]
+        Some("zerocopy-worker") => zerocopy::worker::run_from_args(&args[1..]),
         // NV12 colour self-test (no display/capture needed): convert a known RGBA pattern to NV12
         // on the GPU and compare against a BT.709 limited-range reference. Validates the Tier 2A
         // `PUNKTFUNK_NV12` convert is colour-correct. Prints PASS/FAIL + max Y/U/V error.