ext-stage-dock: full-height left rail — 288px wide, gradient, centered cards

Turn the card-hugging dock into a full-height left rail.

- C++ (extension.cpp/dock_layout.hpp): the surface is now the full OUTPUT HEIGHT
  (hug reverted; content_height helpers/tests dropped) and kDockWidth widened
  240 -> 288 (~20%). Cards unchanged at 224dp. set_visible still hides the rail
  when empty, so it only appears when there are minimized windows.
- RCSS (dock.rcss): body.dock fills the surface (width:100% height:100%) as a flex
  COLUMN scroll container with a `horizontal-gradient(#000000ff #00000000)`
  decorator — fully-opaque black at the left screen edge fading to transparent
  across the wider rail. Cards centered both axes (align-items:center + a
  `div.rail` margin:auto wrapper that vertically centers when they fit and
  collapses gracefully — no flex-center+overflow strand — scrolling when they
  overflow). Scrollbars hidden (scrollbarvertical/horizontal size 0).
- RML (dock.rml): a `div.rail` wrapper around the data-for slot list enables the
  center-or-scroll pattern.

Accepted caveat (quick path): the full-height 288px surface consumes pointer/touch
across the left strip while shown; the deferred input-transparent UiSurfaceSpec
flag is the real fix. Real-seat verified via live RML/RCSS hot-reload.
ext-stage-dock 2/2 green on build + build-asan.

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