comp: add smooth fade transition to active window hint

[DavidDaza2906]

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Focus indicator fade transition (focus_transition_ms)

Partially addresses #989 (adds a smooth transition to the active window hint instead of a temporary hint; happy to extend toward that UX)

Attribution: implemented with AI assistance; the feature design, scope, testing,
and validation are mine (also disclosed in the commit message).

Problem

The focus indicator ("active hint") currently toggles on/off in a single frame. With
focus_follows_cursor enabled and overlapping windows, focus changes feel abrupt/aggressive:
the border pops in and out on every hover.

Approach

This adds an opt-in fade of the focus indicator on activation changes:

• New compositor option focus_transition_ms in CosmicCompConfig (default 0 = exact
  current behavior, no extra allocations or render work).
• Shell::update_active now returns which windows changed activation state, and each
  CosmicMapped stores its transition state (from_alpha, direction) so re-triggered
  transitions resume from the currently displayed alpha instead of restarting — rapid
  focus changes (e.g. double events from a single click) degrade to a continuous fade
  instead of flickering.
• A single repeating calloop timer drives ~60fps redraws while any transition is active
  and self-cancels once they all finish (rendering one final settled frame).
• The floating and tiling render paths share the interpolation via
  CosmicMapped::focus_indicator_{alpha,visible}:
  • the newly focused window fades in,
  • the previously focused window fades out (cross-fade).

Tiling resize/swap indicators keep their existing behavior; only the alpha of the
focus indicator is interpolated.

Notes

• Default 0 means byte-identical rendering to before for users who don't opt in
  (the interpolation branch is skipped entirely).
• The transition works for both pointer-driven focus changes (focus_follows_cursor)
  and keyboard-driven ones, and also fires on workspace switches via refresh_focus.
• Groups/backdrops and swap-mode indicators are intentionally left at full alpha.
• The forced-redraw timer relies on schedule_render, which is a no-op on the winit
  backend (see State::schedule_render); the fade animates on X11 and KMS backends.
  Happy to adapt if upstream prefers a different redraw-scheduling mechanism.

Testing

• cargo check/build on Arch (rustc 1.97, smithay git e3d461a).
• Manual testing nested (X11 backend inside a COSMIC session): open several windows,
  change focus with Super+arrows / focus-follows-cursor / clicks, observe the cross-fade
  of the hint.
• Verified that with focus_transition_ms: 0 rendering is unchanged.
• Pixel-level verification with an artificially long transition (30s): the hint
  border renders at intermediate alpha during the transition (e.g. RGB
  57,102,109 mid-fade) and reaches the exact full-accent color (99,208,223) when it
  completes, for both fade-in and fade-out directions.
• Repeated rapid clicking (the initial implementation flickered here) was fixed by the
  resume-from-current-alpha model and verified clean.

[leviport]

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[DavidDaza2906]

Sorry about that — I accidentally replaced the template when writing the description. It's filled out now, and I've also added the AI disclosure to the commit message itself, as required by the first checkbox.

[Drakulix]

This change would need to go through @pop-os/ux first, to determine if we want that feature. The referenced issue doesn't indicate that this happened yet.

* A single repeating calloop timer drives ~60fps redraws while any transition is active
  and self-cancels once they all finish (rendering one final settled frame).

Code-wise this is also not acceptable. We already have animation logic, this should re-use that.