ext-stage-dock: thumbnail fully covers the card (fix right-edge placeholder sliver)

Real-seat pixel sampling showed a ~2px vertical sliver of the slot placeholder
(#2e2e32) on the card's RIGHT edge only (full height; other edges flush). A magenta
diagnostic background on the thumb proved the gap was NOT a transparent texture edge
(it stayed placeholder-colored, not magenta) — the thumb BOX was ~2px short on the
right (RmlUi box rounding anchored top-left). The preview texture import is correct.

Fix: the full-bleed thumb now overscans the slot by -2dp on all sides; the rounded
overflow:hidden slot clips the overscan, so the image covers the whole rounded card
with no placeholder edge on any side and corners stay rounded. RCSS-only.

Verified real-seat: image reaches all four edges, zero placeholder sliver.
ext-stage-dock 2/2 green on build + build-asan. Also records the transparency/
card-redesign milestone in tasks.md.

2 files changed, 19 insertions, 4 deletions

diff --git a/packages/ext-stage-dock/src/extension.cpp b/packages/ext-stage-dock/src/extension.cpp
index bd643ba..caac456 100644
--- a/packages/ext-stage-dock/src/extension.cpp
+++ b/packages/ext-stage-dock/src/extension.cpp
@@ -235,12 +235,20 @@ div.slot {
     animation: slot-enter 0.16s cubic-out 1 normal;
 }
 div.slot div.thumb {
+    /* OVERSCAN the slot by 2dp on every side: RmlUi resolves the inset/percent
+       box anchored top-left, leaving the thumb ~2px short on the RIGHT so the
+       #2e2e32 slot placeholder peeked through there (diagnosed real-seat: the
+       sliver sampled as the placeholder, not the thumb). A negative inset makes
+       the thumb a few dp LARGER than the slot; the slot's rounded overflow:hidden
+       clips the overscan to the rounded card, so every edge is fully covered with
+       no placeholder peek and the corners stay rounded. cover/center keeps the
+       slightly-larger box fully covered + centered. */
     display: block;
     position: absolute;
-    left: 0;
-    top: 0;
-    right: 0;
-    bottom: 0;
+    left: -2dp;
+    top: -2dp;
+    right: -2dp;
+    bottom: -2dp;
 }
 div.slot span.title {
     /* Title overlay INTENTIONALLY NOT RENDERED (user decision): the card is
diff --git a/tasks.md b/tasks.md
index 1ca8383..3a532b8 100644
--- a/tasks.md
+++ b/tasks.md
@@ -40,6 +40,13 @@ real-seat feel pending):**
   through; cards keep their panel), surface hugs the card stack (no full-height
   input capture), and re-minimize-after-empty fixed (stale `focused_`: restore now
   sets it directly since a non-defocused window's `focus()` is a seat no-op). (661166a)
+- CARD = ROUNDED THUMBNAIL (REAL-SEAT VERIFIED): the card IS the window preview,
+  rounded on all four corners — a full-bleed `image(... cover center)` decorator on
+  a child of a rounded `overflow:hidden` slot (RmlUi won't clip an element's OWN
+  decorator to its OWN radius → decorator lives on the clipped child). First use of
+  the substrate's RmlUi clipping path (scissor + stencil clip-mask); kernel verified
+  it correct + added 4 regression tests (6519ebf). Title overlay parked
+  (`display:none`, binding kept) for a later text redesign — user's call. (a743f44)
 
 **NEXT (needs user):**
 1. REAL-SEAT feel check (covers c2+d1): `~/start-unbox.sh -s foot`, Super+M minimizes