feat: cache bottom-panel geometry; resolve_bottom_panel_zone (#418)

[JDonaghy]

Summary

• GTK: terminal panel click handlers should use cached hit regions from draw #418 main feature — Both GTK click handlers (mouse-down, mouse-drag) and the TUI mouse handler each recomputed the bottom panel's top-y and zone-row boundaries from scratch (~20 lines of chrome/status/wildmenu math per site, including a pixel-exact maximize snap that had to match draw.rs by hand — three copies, three drift opportunities). Now: paint-time writes a single BottomPanelGeometry { top_y, height, row_h }; click handlers call Engine::resolve_bottom_panel_zone(y) -> Option<BottomPanelZone> and dispatch on TabBar / Toolbar / Content { row_offset }. Net -125 lines across the two backends. Sidesteps the PanelChromeDesc row-vs-pixel discrepancy (quadraui: PanelChromeDesc is row-based but GTK chrome is sub-row (pixel mismatch) #168) for the click path — paint/click agreement is guaranteed by construction.
• Drive-by fix: — quadraui::ContextMenuItem gained checked/key_equivalent/submenu fields; develop wasn't compiling. Added ..Default::default() to the 4 literals in src/render.rs. Smallest possible unblock; the struct derives Default and the docstring explicitly recommends struct-update syntax.
• CLAUDE.md docs: — New step 1 in the dev workflow: claim an issue before starting work via gh issue edit <N> --add-assignee @me plus an empty branch push. Multiple agents may run concurrently; an explicit claim signal prevents races. Step 3 adjusted so subsequent pushes to the claim branch are fine (PR-creation gate is preserved).

Closes #418.

Test plan

• cargo fmt && cargo clippy -- -D warnings && cargo test --no-default-features --lib && cargo build — all green locally before pushing
• GTK terminal click parity (non-maximized): open the terminal panel, click the tab bar (TERMINAL / DEBUG CONSOLE), click toolbar buttons (split / maximize / close / + new tab), click in the content area and drag-select text
• GTK terminal click parity (maximized): maximize the terminal panel, repeat all of the above — verify the panel top still resolves correctly under the pixel-exact maximize snap
• GTK split divider: with terminal split active, click the divider (should start drag) and the panes on either side (should switch focus)
• TUI terminal click parity: same checklist as GTK — tab bar, toolbar buttons, content selection, scrollbar column drag in the scrollback
• TUI terminal-debug switch: with both terminal and debug output present, click between TERMINAL and DEBUG CONSOLE in the tab bar
• Click below the panel: verify clicks on the status bar / wildmenu / debug toolbar rows don't wrongly resolve to terminal content (the new height bound in BottomPanelGeometry)

🤖 Generated with Claude Code

[JDonaghy]

Rebased onto develop, fixed three issues:

1. BottomPanelGeometry — replaced uniform row_h with explicit toolbar_y / content_y / content_row_h offsets so backends with non-uniform row heights get correct zone detection.
2. TerminalCopySelection RefCell crash — same double-borrow pattern fixed in refactor: route GTK clipboard read/write through engine callbacks (#417) #419, reintroduced by the PR's old code surviving rebase.
3. ContextMenuItem compile error — removed ..Default::default() (struct doesn't derive Default).

Pre-existing TUI terminal split issues filed as #428. Terminal click dedup follow-up filed as #429.