ext-stage-dock: transparent strip, surface hugs cards, fix re-minimize after empty

Builds on the kernel per-pixel-alpha + set_size-resize capabilities.

- Transparent strip: body.dock background #1c1c1ee6 -> transparent, so the
  windows beneath show through everywhere the cards don't cover; cards keep their
  solid #2e2e32 panel. data-attr-src preview, Noto Sans, and the d1 slot-enter
  animation are intact.
- Surface hugs the card stack: height = surface_height(count) =
  max(1, 2*pad + count*card + (count-1)*gap) (0->1px hidden, 1->140, 2->272, …),
  never the full output height, so the transparent area doesn't needlessly
  capture input. The empty dock is a positive 1px hidden placeholder (the substrate
  rejects 0 geometry); grows/shrinks via set_size on minimize/restore.
- Fix: re-minimize after the dock empties was a no-op. do_restore relied on
  on_toplevel_focused re-firing, but a restored window was never seat-defocused,
  so focus() is a seat no-op and the event never fires — leaving focused_ stale,
  so the next Super+M's focused_ guard rejected it (a new toplevel mapping
  unstuck it). Fix: set focused_ = tl directly in restore. No kernel change.

Tests: new policy cases (surface_height always positive; hug heights 0/140/272/536)
and a glue minimize->restore->minimize 1->0->1 cycle with has_focused() probe.
ext-stage-dock 2/2 green on build + build-asan (no sanitizer reports). Real-seat
verified: transparent strip, dock visible, cards float, re-minimize works.
Edits confined to packages/ext-stage-dock/.

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