TUI debug sidebar: scrollbar click doesn't reach handle_mouse

[JDonaghy]

Problem

In the activity-bar Debug sidebar (Variables / Watch / Call Stack / Breakpoints), per-section scrollbar clicks don't trigger the click handler in `src/tui_main/mouse.rs`. The user reports that thumb-drag and track-click both fail to scroll the section.

Investigation so far

After Phase B.4 Stage 5c (commit `f8d394f`), the click handler at `src/tui_main/mouse.rs:2641` should arm a `quadraui::DragState::ScrollbarY` with widget id `tui:debug_sidebar:N`. The renderer's thumb-position formula was also fixed to match `dispatch_mouse_drag` math (`scroll * sec_height / total_items` instead of the previous one-off `/(total - sec_height)`).

A debug-log trace (`vcd --debug /tmp/vcd.log`) showed:

• Clicks were captured at `col=5..11`, never near the rendered scrollbar column (`sb_col=31` in that session).
• A second test with a single deliberate scrollbar-track click produced zero `debug-sidebar` log entries — the click never reached `handle_mouse`'s debug-sidebar branch at all.

Hypotheses

1. The scrollbar isn't visually rendered in the user's setup despite the heights/item-count condition being met (renderer bug).
2. A gating check earlier in `handle_mouse` swallows the event before reaching the debug-sidebar block.
3. Coordinate mismatch — terminal mouse-event column doesn't match the visible scrollbar column.

Repro

1. Run `cargo run --bin vcd -- --debug /tmp/vcd.log `.
2. Set breakpoints / start a debug session so a section overflows its allocated rows.
3. Click on the visible scrollbar column at the right edge of an overflowing section.
4. Observed: nothing scrolls. Log shows no `debug-sidebar` entries.

Files

• `src/tui_main/mouse.rs:2611-2700` — `TuiPanel::Debug` click handler.
• `src/tui_main/panels.rs:3360-3380` — section scrollbar renderer.
• `src/tui_main/mouse.rs:62-133` — `apply_scrollbar_drag` apply-side router.

Acceptance

Click anywhere on the visible thumb or track of any debug-sidebar section should scroll that section, with thumb-grab preservation matching every other migrated scrollbar.

[JDonaghy]

Fixed by #307 Batch 2 — debug sidebar sections now register as ScrollSurface entries at paint time. Scrollbar clicks route through dispatch_click, which handles thumb drag initiation and track page-up/page-down.