drm: convert the last panicking spawn, and resolve geometry outside the lock

The spawn conversion in the previous commit missed one. `query_displays` still
used `std::thread::spawn`, which panics when a thread cannot be created, and it
is reached from both `get_capturer_info` and `warm_availability` - so the panic
would land on the capture-build path rather than being reported as the failed
probe every caller already handles. There are now none left in the two DRM
files.

Worth writing down how it survived a pass whose whole purpose was to find it:
the previous commit enumerated the siblings with a grep piped through `head`,
there were eleven matches, and `head` printed ten. The one it cut is the one
that was missed. Same shape as a build log read through `tail` and a `find`
given `-xdev`: the tool truncated the survey and the survey looked complete.
When enumerating sites for a class fix, do not pipe the enumeration.

Also, `get_capturer_for_display` resolved the advertised DRM geometry while
holding the `CAP_DISPLAY_INFO` read guard. That lookup runs a compositor output
roundtrip, and `clear()` takes the write guard on every capturer teardown -
which is what is happening when a display is demoted or flapping, i.e. exactly
when this path runs. The value does not depend on anything inside the guard, so
it is resolved before taking it.

And the security doc listed the unattended package's `Conflicts`/`Replaces` but
not its `Provides: rustdesk`, which is the field that lets a third-party package
depending on `rustdesk` be satisfied by the consent-free variant. An operator
auditing that metadata needs all three.

docs/DRM_CAPTURE_SECURITY.md

@@ -202,7 +202,9 @@ presents) but reuses RustDesk's own hardened IPC.
   byte-identical to upstream. Build it explicitly with
   `python3 build.py --flutter --drm` (Linux only).
 - **Separate opt-in package.** A `--drm` build ships as a distinctly named
-  `rustdesk-unattended-wayland` package (Conflicts/Replaces `rustdesk`), so
+  `rustdesk-unattended-wayland` package (Conflicts/Replaces/**Provides** `rustdesk` --
+  `Provides` is what lets a third-party package that depends on `rustdesk` be satisfied by the
+  consent-free variant, so it belongs in an audit of this metadata), so
   enabling consent-free capture is an explicit install choice.
 - **Bundled library, no capabilities.** The package installs the versioned
   `libdrmtap.so.0.<minor>.<patch>` plus a `libdrmtap.so.0` soname symlink under