fix: wire the mouse side buttons to back and forward

[fgrehm]

Problem

The README has always documented that the mouse's side buttons retrace the visited directories, and Disktree::on_mouse_down had the two MouseButton::Navigate arms for exactly that. But the mosaic registered its mouse-down listeners per button, for Left and Middle only, so buttons 8 and 9 never reached on_mouse_down and the arms were dead code. The keys (alt ← alt →, ⌘[ ⌘]) and the header < / > buttons worked, which is why this went unnoticed.

Fix

• Replace the two per-button registrations on the mosaic with on_any_mouse_down, so every button reaches on_mouse_down and the existing Navigate arms work. One listener instead of four, and the per-button form would have needed a registration per button the platform names.
• Guard the Navigate arms on Screen::Explore, matching the alt-arrow path. On the review screen the marked list is not somewhere history should move.
• Add a treemap debug selector, so the harness test can aim at the mosaic the way a person aims with the pointer.
• The help screen and the hover card on < / > named only the keys, so the in-app bindings were narrower than the README's. Both now mention the side buttons.

Verification

cargo xtask ci is green: rustfmt, clippy with the workspace's pedantic and nursery lints at -D warnings, then 55 window-harness tests and 105 core tests.

The new test mouse_side_buttons_go_back_and_forward drives the real window harness: it descends twice, presses button 8 over the mosaic and asserts the crumbs retrace, presses button 9 and asserts they return, then enters the review screen and asserts a further press 8 changes nothing.

That test synthesizes the MouseDownEvent, so it proves the wiring, not that a compositor delivers button 8. gpui-pre does map them: buttons 8/9 on X11, BTN_BACK/BTN_SIDE and BTN_FORWARD/BTN_EXTRA on Wayland, and XBUTTON1/XBUTTON2 on Windows. There is no mapping in gpui-pre-apple, so side buttons stay macOS-dead, where the keys and the header buttons are the whole story.

Checked by hand on Linux (Hyprland) with a real mouse: buttons 8 and 9 retrace the history from over the mosaic, and do nothing on the review screen.