7c-2: draw_corner_dot earned; RFC — cce-files adopts the affordance

The second consumer's findings: the dot-drawing helper is real shared code
(both hosts draw it now), a PlateDock container is NOT yet earned (one
state field sufficed), press arming is drag-host-specific, and per-app
event dispatch must be checked (files keys on releases — its routed path
consumes presses before the Application hook). The detach CSD packaging
and cce-cloud silhouette questions carry to the first non-designer detach.

 docs/rfc-core-rebuild.md | 26 ++++++++++++++++++++------
 src/widget/plate_dock.rs | 13 +++++++++++++
 2 files changed, 33 insertions(+), 6 deletions(-)

diff --git a/docs/rfc-core-rebuild.md b/docs/rfc-core-rebuild.md
index 8812c83..350a021 100644
--- a/docs/rfc-core-rebuild.md
+++ b/docs/rfc-core-rebuild.md
@@ -2525,12 +2525,26 @@ Constraint respected: **each crate still builds standalone** — the new core is
       application, and the detach process spawn stay designer policy. Verified live:
       corner menus (Collapse/Detach; Reattach-only stubs), the collapse→stub→expand
       cycle, and dock-drag arming+drop all behave identically through the delegation.
-    - **7c-2 — pending.** A second consumer adopts the affordance (cce-mail's
-      list/detail split or cce-files' panes are the candidates) — that adoption decides
-      whether a `PlateDock` container type earns its place, whether the corner control
-      becomes a drawable helper, and how `standard_csd`/`take_window_action` package the
-      detached-window CSD for hosts without the designer's custom chrome. The cce-cloud
-      overlay-silhouette question from 7b-2 also lands here.
+    - **7c-2 — DONE (2026-08-25): cce-files is the second consumer.** Its preview pane
+      carries the corner control; Collapse narrows every page's pane column to a Preview
+      title stub (the list keeps the freed width — the feature's actual value), the
+      stub's control restores the prior split fracs, and the divider/wheel are inert
+      while collapsed. The adoption answered the open API questions:
+      - `draw_corner_dot(pc, center, emphasized)` EARNED — both hosts drew the identical
+        dot; the designer now uses it too.
+      - A `PlateDock` CONTAINER did NOT earn its place: one `PlateDockState` field
+        sufficed. Revisit only if a host manages many dockable plates outside its own
+        state arrays.
+      - Press arming was designer-specific after all: it exists to disambiguate click
+        from dock-DRAG, and a single pane has nowhere to dock — files opens the menu
+        directly. Moreover, files keys the whole interaction on mouse RELEASES: its
+        routed-widget path consumes left PRESSES before the `Application` hook (only
+        releases reliably arrive there), a per-app dispatch reality any adopter must
+        check first. The designer's release-opened menu means the two feel identical.
+      - The detached-window CSD packaging question REMAINS OPEN (files has no detach
+        model; `standard_menu` is called with `can_detach = false` and the rows never
+        appear). It lands with the first non-designer detach implementation, as does the
+        cce-cloud overlay-silhouette question from 7b-2.
 
   Not started; no code moves before 7a's alias sweep is reviewed, and stages land in order —
   7b is the value (one paint path, plates compositional), 7c is the payoff feature.
diff --git a/src/widget/plate_dock.rs b/src/widget/plate_dock.rs
index 5b9bb5c..a6ac524 100644
--- a/src/widget/plate_dock.rs
+++ b/src/widget/plate_dock.rs
@@ -91,6 +91,19 @@ pub fn press_becomes_drag(press: (f32, f32), cursor: (f32, f32)) -> bool {
     (cursor.0 - press.0).abs().max((cursor.1 - press.1).abs()) > DRAG_THRESHOLD
 }
 
+/// Draw the corner control at `center`: the plate-border-colored dot,
+/// enlarged when `emphasized` (hovered, or its menu is open). Earned by the
+/// second consumer (RFC 7c-2) — both hosts drew the identical dot.
+pub fn draw_corner_dot(
+    pc: &mut crate::scene::paint::PaintCtx,
+    center: (f32, f32),
+    emphasized: bool,
+) {
+    let r = if emphasized { CORNER_R * 1.15 } else { CORNER_R };
+    let fill = crate::color::plate_border_color().unwrap_or([0.55, 0.58, 0.66, 0.85]);
+    pc.circle(center.0, center.1, r, fill);
+}
+
 /// The standard corner-menu rows for a plate in `state`. A detached pane's
 /// stub offers ONLY Reattach (collapsing it would mean nothing); otherwise
 /// Collapse/Expand per state, then Detach when the host allows it. The host