feat(ribbon): collapse overflowing panels to flyout buttons, anchor dropdowns to their widget

Third stage of the adaptive tool area (shrink -> compact -> collapse).
When even the compact panels don't fit, DensitySwap picks a new
CollapsePanels widget that keeps a left prefix of panels inline and
collapses the rest to buttons; clicking a button opens the full panel as
an overlay flyout anchored below it (closes on outside press, mirroring
color_select's Floating).

A collapsed panel's button shows a live representative tool — the panel's
last-used tool (tracked per panel in Ribbon::note_panel_tool on every
tool click), defaulting to its first — rendered as a large icon. To
support this, render_small now also renders Large* items small and
render_large now renders plain Tool/Dropdown items large. Large dropdowns
render as icon + label + a bottom arrow strip via a shared
render_large_dropdown helper, used in both the full ribbon and collapsed
panels.

Ribbon dropdowns (tool dropdowns + undo/redo history) now anchor their
overlay directly below their widget at any size: a transparent PosReport
wrapper records each dropdown button's screen bounds into a thread-local
map on draw, and the overlays read it via Ribbon::dd_anchor instead of the
layout-estimating compute_*_left helpers.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -1403,6 +1403,8 @@ pub enum Message {
     SnapClearAll,
     /// Toggle a ribbon dropdown open/closed.
     ToggleRibbonDropdown(String),
+    /// Toggle a collapsed ribbon panel's flyout open/closed (by panel title).
+    ToggleRibbonPanel(String),
     /// Close any open ribbon dropdown (click-catcher outside the panel).
     CloseRibbonDropdown,
     /// User selected a specific item from a ribbon dropdown.