Harden cross-desktop fallback and packaging metadata

[Zakkaus]

Summary

Hardens the cross-desktop fallback path and fills in the packaging metadata a
Linux desktop app is expected to ship. Comes out of a cross-DE / GPU /
OSS-hygiene audit of the code against the Gentoo ebuild in
gentoo-zh/overlay#10842.

Two classes of change: runtime correctness (detect layer-shell support and
Qt-ABI compatibility at runtime instead of guessing from the desktop name), and
packaging hygiene (AppStream metainfo, man page, PEP 639 license, desktop
entry keys, an explicit blur build option).

Runtime correctness

X11 gate (native.py). On an X11 session the bridge .so still dlopens
(its Qt/Wayland deps are present regardless), but every call no-ops on an xcb
surface -- which silently disabled the top-most/drag fallback the overlay is
supposed to use there. The controller now refuses the bridge up front unless the
platform is Wayland, so the fallback actually runs.

Runtime layer-shell probe (koto_has_layer_shell). New C export does a
one-shot wl_registry probe for zwlr_layer_shell_v1. This is authoritative
where the desktop-name check is not: it catches Weston, Cinnamon, and GNOME
under any XDG_CURRENT_DESKTOP value, and avoids the Budgie/wlroots false
positive. It is hasattr-guarded on the Python side, so an older bridge without
the symbol still loads and falls back to the existing name check.

Qt-ABI handshake (koto_layer_qt_version). The bridge links Qt QPA/private
API, which carries no cross-minor ABI guarantee. _load() now refuses a bridge
built against a different Qt minor than the running PyQt6. This only bites a
hand-built/mismatched wheel; a distro build shares one system Qt. Also
hasattr-guarded.

main.py. Drops the dead QT_AUTO_SCREEN_SCALE_FACTOR env knob -- a Qt5-ism
Qt6 never reads (it misleads readers into thinking scaling is pinned). Keeps
QT_SCALE_FACTOR, which does pin the device pixel ratio.

Packaging

• CMakeLists: blur is now an explicit KOTONOHA_ENABLE_BLUR option (default
  ON, FATAL_ERROR if requested without wayland-scanner) rather than a silent
  find_program degrade. This removes the "automagic" dependency pattern
  packagers reject. Default ON is safe because the build already hard-requires
  the wayland-client dev package, which ships wayland-scanner. Adds a
  KOTONOHA_INSTALL_LICENSE gate and a Qt-version STATUS line.
• pyproject: PEP 639 license = "MIT" + license-files = ["LICENSE"]
  (emits License-Expression/License-File, metadata 2.4); trims build-only
  sources (*.cpp, *.sh, protocols/*.xml) from the wheel.
• AppStream: packaging/dev.locez.kotonoha.metainfo.xml so the app is
  visible in Discover/GNOME Software. project_license is MIT (the LGPL is
  dependency linkage, not this component's source license); launchable points
  at the installed kotonoha.desktop; no <screenshots> until a hosted image
  and a tagged release exist.
• Man page: packaging/kotonoha.1 (--port/-p, -v/--verbose, default
  port 28745).
• Desktop entry: Version=1.5, StartupNotify=false (tray app, no window
  on launch), StartupWMClass=kotonoha (matches the app_id Qt derives from
  setApplicationName).
• All distros ship the new files: the metainfo and man page are installed by
  the Debian (debian/install) and Fedora (%install/%files) packaging too,
  not just the external Gentoo ebuild. The deb/rpm CI smoke jobs assert both
  landed.
• Dropped a duplicate asset: assets/icons/icon.png was byte-identical to
  the default assets/icon.png; discover_icon_paths already deduped it out by
  digest at runtime, so it never showed in the picker -- removing it trims ~858K
  from the repo and wheel.
• README documents the KOTONOHA_ENABLE_BLUR / KOTONOHA_INSTALL_LICENSE
  build options.

Capability matrix

Session	Overlay mode	Blur
KDE Plasma (Wayland)	layer-shell (float above fullscreen)	yes (KWin)
wlroots (Sway/Hyprland, Wayland)	layer-shell	no (no KWin protocol)
GNOME/Mutter (Wayland)	top-most ordinary window (probe returns 0)	no
Weston / Cinnamon (Wayland)	top-most ordinary window (probe returns 0)	no
X11 (any WM)	top-most window via _NET_WM_STATE_ABOVE	no

GPU vendor (NVIDIA/AMD/Intel) is irrelevant here: the overlay is a wl_shm
raster QWidget surface with no GL/EGL/QtQuick path, so no vendor-specific env
tuning applies. The "EGL not available" log line is benign noise.

Tests

tests/test_native.py covers the X11 gate, the runtime probe
(present / absent / old-bridge-without-symbol), and the real-bridge Qt
handshake. tests/test_native_packaging.py updated for the new wheel.exclude.
Full suite: 357 passing.

Not in this PR

The corrected kotonoha-9999.ebuild for gentoo-zh/overlay#10842 is handled
separately in that repo (adds dev-util/wayland-scanner BDEPEND, qtbase:6[gui],
layer-shell-qt:6, LICENSE="MIT LGPL-2.1+", installs the metainfo + man page).
Wiring KOTONOHA_INSTALL_LICENSE=OFF into the wheel build is deferred until the
deb/rpm specs migrate their license declarations to .dist-info/licenses/.