Add drop shadows and shrink the resize grab margin that was eating content clicks

Two independent daily-driving gaps closed in one pass, both from MISSING.md
and live user feedback:

Drop shadows (general.shadows, default true). Reuses the exact "bitmap
drawn outside geometry, cached like the border" technique border_strips/
render_border_top already established - decoration::shadow_bitmap rasterizes
a linear alpha falloff (Chebyshev/square-ring distance, not a true blur --
no blur primitive exists without a GPU shader, and the udev backend's
PixmanRenderer is software-only) from SHADOW_MAX_ALPHA (90/255, deliberately
subtle) at the window's own edge down to fully transparent SHADOW_SIZE (12px)
out. Cached in CompState::shadow_buffers, rebuilt at the same trigger points
as border_top_decorations (redraw_decoration_buffer), for the identical
damage-tracking reason: a fresh Id every frame means OutputDamageTracker
never finds a previous-frame match. No shadow for a maximized or fullscreen
window, matching the Hyprland/GNOME convention MISSING.md measures against.

Resize grab margin: 10px -> 6px (general.resize_margin, now configurable,
same call-site-count-preserving change as threading a new parameter through
one indirection point: ResizeEdge::hit_test's only production caller is
WindowManager::hit_test, so this didn't need touching every backend despite
hit_test being shared verbatim across X11/Wayland/Windows/macOS). Reported
live: ordinary clicks near any window edge - a link near a browser's edge,
a button near a panel's edge - regularly registered as a resize-edge grab
instead of reaching the client, not just an occasional near-miss, because
the 10px band was measured inward from the client's own content rect. 6px
stays comfortably grabbable while giving content back most of its edge.

Verified: cargo build --workspace (all 9 crates including the windows/macos
stub backends), cargo clippy --workspace (0 new warnings), cargo test across
core/wayland/config/x11 (188 tests, 0 failed). Shadow rendering verified at
the render-element level live in a nested session (correct geometry, alpha,
buffer contents) - grim/screencopy itself turned out to route through
winit.rs's separate capture_offscreen path, which only ever drew
`decorations` (titlebars), never borders or shadows, so screenshots taken
this way have never shown either; a real gap, not fixed in this pass.

0 files changed, 0 insertions, 0 deletions