drm: address the phase-2 split review

1- do not depend on the libdrmtap-sys crate for the pin: its build.rs statically
compiles the whole libdrmtap C tree and a CAP_SYS_ADMIN helper and links
-ldrm/-lseccomp/-lcap, which defeats the runtime-dlopen model. keep drm a pure
dlopen backend and pin the .so by the build.py DRMTAP_REF release tag, guarded by
a strict vX.Y.Z regex. drops the now-moot Cargo.lock freshness CI checks.
2- render-node-less consumers no longer lose the stream: the --server signals
need_cpu on DrmStart when it cannot open a convert context, and the --service
streams the CPU-converted frame path for that connection instead of a dma-buf fd
the consumer cannot detile (which used to fall through to a PipeWire path nobody
can approve on an unattended seat).
3- mark PipeWire initialized only after every per-display capturer is created, so
a partial failure retries instead of the flag falsely reporting a complete init.
4- reject a degenerate (zero width/height) or short CPU frame before it reaches
PixelBuffer::new (which derives stride as data.len()/height, dividing by zero).
5- keep the export-ledger epoch at DRM_DISPLAY_GENERATION so a hotplug invalidates
cached buffers (elision stays off until the recycled-fb_id inode case is handled).
6- validate the udev uevent source (kernel nl_pid, multicast) with recvmsg so a
local process cannot unicast a spoofed drm-change event to the root listener.

build.py

@@ -335,11 +335,11 @@ def ffi_bindgen_function_refactor():
 # libdrmtap is fetched at build time by cloning the rustdesk-org fork at a pinned
 # ref — the same way rustdesk sources its other native build deps (vcpkg,
 # flutter_rust_bridge, ...), rather than carrying a git submodule. The ref is an
-# EXACT release tag (vX.Y.Z), NOT a moving branch: the bundled runtime .so version must
-# match the `libdrmtap-sys = "=0.4.13"` crate pinned in libs/scrap/Cargo.toml (the dlopen
-# loader only checks ABI-major, so a minor skew between the two artifacts would go undetected).
-# Bump this tag and the crate pin together. Override the repo/ref via env (DRMTAP_REPO /
-# DRMTAP_REF) for local testing or another fork.
+# EXACT release tag (vX.Y.Z), NOT a moving branch, and it is the ONLY pin for the
+# drm backend: rustdesk dlopens this .so at runtime and does not depend on the
+# libdrmtap-sys crate (whose build.rs would statically link the C tree, a helper and
+# libdrm/seccomp/cap). Override the repo/ref via env (DRMTAP_REPO / DRMTAP_REF) for
+# local testing or another fork.
 LIBDRMTAP_REPO = os.environ.get('DRMTAP_REPO', 'https://github.com/rustdesk-org/libdrmtap')
 LIBDRMTAP_REF = os.environ.get('DRMTAP_REF', 'v0.4.13')