fix(border): debounce border redraws adaptively to prevent flicker without lag

[lekifier]

Problem

When switching spaces with yabai's instant (scripting-addition) switch, the focused window's border visibly flickers — it gets redrawn several times in quick succession. This does not happen with macOS's native (animated) space switching. Reported in #79.

Root cause

On a space change, a window is hit by a burst of events — Unhide, Level, Reorder, and Focus — each of which calls border_update(), and each redraw is a visible flash. The focused border is therefore repainted ~4-5 times within ~50ms.

Fix

An adaptive per-border debounce in border_update():

• A constant 80ms debounce (my first attempt) kills the flicker but makes the border visibly lag behind fast focus changes, because every update reschedules the wait. So the delay is now context-dependent:
• borders already receives EVENT_SPACE_CHANGE (space_handler in events.c); when it fires, a 500ms transition window opens.
• Inside the window, updates are debounced by 80ms, collapsing the burst into a single redraw with the final state (anti-flicker).
• Outside the window, updates are debounced by only 5ms, so the border tracks focus changes almost instantly.
• Each border keeps an incrementing update_generation counter; a newer request supersedes older scheduled ones in both modes.

Also drops the unreachable async path left in border_update() (border_update_async_proc).

Tested with yabai instant space switching and fast window cycling: no flicker on space change, border tracks focus immediately otherwise.

Fixes #79