feat: add OBS recording engine and validation probe

clipbin-obs starts the pinned libobs on a dedicated thread, opens exactly the
bundled plugin allowlist, and records official capture sources through one
hardware encoder into keyframe-aligned Matroska segments. Status reaches
Recording only once the source has a picture and the segment grows on disk.

clipbin-probe lists and verifies hardware encoders and records from a real
capture source, printing status, health and segment events for validation.

.cargo/config.toml

@@ -0,0 +1,6 @@
+# Development builds use the pinned OBS runtime assembled by scripts/obs/.
+# Packaged builds find the runtime relative to the executable instead.
+[env]
+CLIPBIN_OBS_RUNTIME_DIR = { value = "target/obs-runtime", relative = true }
+# Linux: link libobs from the bundled runtime rather than a system OBS.
+PKG_CONFIG_PATH = { value = "target/obs-runtime/linux-x86_64/lib/pkgconfig", relative = true }