TUI ext panel: scroll wheel doesn't scroll the panel (scrollbar drag works)

[JDonaghy]

Symptom

In an extension-provided sidebar panel with more items than fit in the viewport:

• Dragging the scrollbar thumb scrolls the panel correctly.
• Scrolling the mouse wheel while hovered over the panel does not scroll.

Repro

1. Open an extension panel (e.g. `git_insights`) with enough items to need scrolling.
2. Hover the mouse anywhere over the panel body.
3. Scroll the wheel — list does not scroll.
4. Drag the scrollbar thumb in the rightmost column — scrolls correctly.

Cause (suspected)

The wheel-dispatch path is registered correctly:

• `src/tui_main/panels.rs` pushes a `ScrollSurface { id: "ext_panel:sb", bounds: }` each frame.
• `src/tui_main/mouse.rs:1233` calls `quadraui::dispatch_scroll(modal_stack, &surfaces, point, delta)`.
• `mouse.rs:1271` matches `"ext_panel:sb"` and adjusts `engine.ext_panel_scroll_top`.

So either:

1. `dispatch_scroll` isn't finding the surface for the mouse position (bounds mismatch, surface cleared too early in the frame, or the modal_stack is intercepting), or
2. The match arm fires but `scroll_top` clamp is wrong.

Worth instrumenting `dispatch_scroll` output and `ext_panel_scroll_top` before/after.

Repro environment

Smoke-tested on (terminal emulator). Filing follow-up to #476 / PR #482 — that migration did not touch the scroll surface registration or the wheel dispatch, so the behavior predates it. (The legacy renderer registered the exact same surface with the same bounds.)