Draw a cursor, handle the lid, and everything a real config needs

Groundwork for actually daily-driving this: porting the user's Hyprland
config exposed what srdwm couldn't yet express, and testing on a bare TTY
exposed something worse.

A visible mouse cursor. Nothing drew a pointer at all - on a bare TTY the
mouse was simply invisible. It hid because the nested backend runs inside
another compositor, which draws a cursor over srdwm's window; only the DRM
backend, i.e. the actual session path, was affected. A built-in arrow is now
composited above everything on the output the pointer is on. It's a
reviewable ASCII bitmap rather than an XCursor theme: a cursor that is always
present beats a prettier one that sometimes isn't, the same reasoning as
decoration.rs's font fallback. Client-set cursor surfaces and named shapes
are still not rendered, so an app asking for an I-beam gets the arrow.

Lid switch. libinput switch events are handled and surfaced to config as
srd.on("lid_closed"/"lid_open", fn), so closing the lid can lock and suspend
instead of doing nothing.

Config-driven additions, each needed by a binding in the ported config and
none of which existed: fullscreen (Window.fullscreen was a dead field --
declared, never read or written), directional window move that swaps with
the neighbour and reorders the stack so tiling follows, focus cycling,
modifier+drag to move/resize anywhere in a window rather than only by the
titlebar, modifier+scroll to change workspace, and 8 XF86 media/power
keysyms taken from the system's own XF86keysym.h. The keysym tables are
hand-maintained in both directions and a key missing from either fails
silently, so a round-trip test now covers every one the configs bind.

Verified in the QEMU VM: a bare-TTY screendump shows a recognisable arrow at
the pointer position (113 white fill + 58 black outline pixels at screen
centre, where the pointer starts).

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