feat(ui): scroll the focused sidebar panel + refine the nav overlay (#21, ADR-0046 DC3 + DC2)

DC3 — navigation-mode scroll: the focused Tables / Relationships panel
scrolls (Up/Down by a line, PageUp/PageDown by its visible-row count).
Per-panel offsets are clamped to content at render time, and the
renderer reports each panel's visible rows for paging — mirroring the
output panel's scroll. render_items_panel / render_relationships_panel
take &mut App, count their rows, and store+clamp the offset before
building the borrowing lines.

DC2 refinement: the expand-on-focus overlay now clears only the sidebar
strip plus a one-column gutter, leaving the base output/input/hint
visible (unchanged) to the right rather than blanking the whole area —
truer to "underneath keeps its layout", with the gutter keeping the
cut-off edge clean (chosen after eyeballing both variants). ADR DC2 and
the overlay snapshot updated to match.

Tests: line/page scroll move only the focused panel and clamp; the
render clamps a past-the-end offset so the last row stays visible.

docs/adr/0046-sidebar-navigation-and-responsive-input-hint.md

@@ -320,14 +320,23 @@ input-editing arms and routes keys per DC3/DC4.
 
 ### DC2 — Expand-on-focus as an overlay
 
-A focused sidebar panel widens to **~40–50 columns**, rendered as an
-**overlay**: the renderer draws a `Clear` over the affected right-column
-region and paints the wide panel on top. The output/input/hint panels
-underneath keep their exact layout — **unused and unchanging** while
-browsing — and are restored by the next frame on exit. This is cheap
-because the renderer is a pure function of `App` state: focus state
-selects the width and the overlay path. (The input underneath is
-inactive in navigation mode, so occluding it is harmless.)
+A focused sidebar panel widens to a **45-column** overlay
+(`NAV_EXPANDED_WIDTH`): the renderer `Clear`s the strip the expanded
+panel occupies **plus a one-column gutter** (`NAV_OVERLAY_GUTTER`) and
+paints the wide panel on top. The output/input/hint panels underneath
+keep their exact layout — **unused and unchanging** while browsing,
+**still visible to the right** of the overlay (just partially occluded
+on the left) — and are restored fully by the next frame on exit. The
+gutter keeps them from butting against the expanded panel's border so
+the overlay edge reads cleanly. This is cheap because the renderer is a
+pure function of `App` state: focus state selects the width and the
+overlay path. (The input underneath is inactive in navigation mode.)
+
+*Implementation note (2026-06-10):* a full-area clear (hiding the base
+panels entirely during browse) was tried first and rejected — leaving
+the base visible is truer to "underneath keep their layout," and the
+one-column gutter resolves the only wrinkle (the panels' left edges
+being cut by the overlay reading harshly without separation).
 
 ### DC3 — Scroll the focused panel; focus highlight