feat(ui): flat filled rectangles for demo overlays (#22, ADR-0047 D4)

Render the keystroke badge and step caption as a solid yellow rectangle
with no border glyphs and a one-cell text margin, instead of a
rounded-border box — deliberately unlike the app's bordered panels so
the demo overlays read as a distinct, eye-catching callout. Shared
fill_overlay_rect helper (borderless Block fill + inset Paragraph).
Snapshots regenerated; ADR-0047 D4 wording updated.

docs/adr/0047-demonstration-overlay-layer.md

@@ -186,12 +186,21 @@ confirmed by test if live `--demo` on Windows is exercised.)*
 
 ### D4 — Both overlays are floating boxes at the output panel's inner bottom-right
 
-The badge and the caption both render as **floating, bordered boxes
-anchored to the inside of the output panel's bottom-right corner**
-(inset one cell from the panel's inner edge), drawn **last over the base
-render** — after modals, so they remain visible while the load-picker
-(the `#24` cast) or any modal is up, and with **no layout reflow**
-(consistent with the modal / nav-overlay precedent; honours R8).
+The badge and the caption both render as **floating, flat filled
+rectangles anchored to the inside of the output panel's bottom-right
+corner** (inset one cell from the panel's inner edge), drawn **last over
+the base render** — after modals, so they remain visible while the
+load-picker (the `#24` cast) or any modal is up, and with **no layout
+reflow** (consistent with the modal / nav-overlay precedent; honours
+R8).
+
+**Flat rectangle, not a bordered box (user decision, post-build).** The
+overlays draw as a **solid yellow rectangle with no border glyphs** and
+a one-cell margin around the text — deliberately *unlike* the app's
+rounded-border panels, so they read as a distinct callout that "stands
+out nicely" rather than as another panel. Implemented with a borderless
+`Block` fill (the `paint_background` mechanism) plus a `Paragraph` inset
+into a one-cell `Margin`.
 
 The top-level `render()` does not currently know the output-panel rect
 (it is computed inside `render_right_column`), so a **new field
@@ -204,19 +213,19 @@ When **both** are present, the **keystroke badge stacks directly above
 the caption box** (both right-aligned in the corner) so they never
 overlap.
 
-**Styling — deliberately high-contrast:** **black text on a yellow
-background**, bold, bordered — hard to overlook, identical in light and
-dark themes (a fixed high-contrast pair centralised in `theme.rs`, not
-theme-derived).
+**Styling — deliberately high-contrast:** **bold black text on a yellow
+fill** — hard to overlook, identical in light and dark themes (a fixed
+high-contrast pair centralised in `theme.rs`, not theme-derived).
 
 **Caption sizing (user-confirmed).** The caption is **word-wrapped to at
 most 3 lines** within a content width of `min(40, output_inner_width −
-6)` columns, ellipsised beyond the third line. So the caption box is
-**3–5 rows** tall (1–3 text rows + 2 border), its height varying with
-the text — a full sentence fits without forcing the author to split it,
-while the 3-line cap keeps it corner-sized. The **badge** box is always
-a single short token (`[TAB]` … `[SHIFT-TAB]`), so it is a fixed **3
-rows** (1 text + 2 border), narrow.
+4)` columns, ellipsised beyond the third line. So the caption rectangle
+is **3–5 rows** tall (1–3 text rows + a one-cell margin top and bottom),
+its height varying with the text — a full sentence fits without forcing
+the author to split it, while the 3-line cap keeps it corner-sized. The
+**badge** rectangle is always a single short token (`[TAB]` …
+`[SHIFT-TAB]`), so it is a fixed **3 rows** (1 text row + the margin),
+narrow.
 
 **Clamping (runda finding).** Stacked, the two boxes are up to 8 rows
 (5 caption + 3 badge); the output panel's inner height is only `Min(5)`,