test(quadraui): TUI MSV paint↔click round-trip harness

[JDonaghy]

Summary

First instance of the new paint↔click round-trip harness pattern for quadraui primitives. Adds 4 tests in quadraui::tui::multi_section_view::tests that paint a MultiSectionView into a ratatui::buffer::Buffer, find painted glyph cells in that buffer, hit_test those exact coordinates against the layout the rasteriser used, and assert the hit identifies the painted-into section.

Also extracts pub fn tui_msv_layout(view, area) -> MultiSectionViewLayout from draw_multi_section_view's body so paint and tests consume one source of truth.

Why this PR exists

Step 3 of the Session 346 course correction. The pattern this PR establishes is the gate for re-attempting #296 (step 6) and migrating SC #282 (step 7), and is the template for future quadraui primitive harnesses (TreeView next, then GTK equivalents).

The bug class the harness catches

Pre-cell_quantum (#297, just merged), fractional EqualShare distributions made paint snap to integer rows via bounds.y.round() as u16 while MultiSectionViewLayout::hit_test consumed raw fractional bounds — clicks at the row paint drew Watch's header on landed in Variables' body. The #296 smoke wave chased that drift across 8 commits. Either of the fractional-distribution tests below would have caught it pre-merge.

Tests added

• header_clicks_land_in_painted_section_under_fractional_distribution — 4 EqualShare sections in 21 cells (5.25 each, the exact worst case). Find each section's title row in the buffer, hit_test there, assert correct Header{section}.
• body_clicks_land_in_painted_section_under_fractional_distribution — same setup, find item rows in each section's body, hit_test there, assert Body{section}.
• scrollbar_column_hits_scrollbar_not_body_when_section_overflows — overflowing section reserves a 1-cell scrollbar gutter; clicks on it must hit Scrollbar, not Body.
• body_and_scrollbar_bounds_never_overlap — geometric exclusion check across multiple sections.

Empirical verification that the harness catches what cell_quantum fixes

Temporarily flipped cell_quantum: 1.0 → 0.0 in tui_msv_layout:

test header_clicks_land_in_painted_section_under_fractional_distribution ... FAILED
test body_clicks_land_in_painted_section_under_fractional_distribution ... FAILED
test scrollbar_column_hits_scrollbar_not_body_when_section_overflows ... ok
test body_and_scrollbar_bounds_never_overlap ... ok

Restored cell_quantum: 1.0: all 4 pass. Harness catches exactly the bug class the structural fix in #297 closed.

Refactor: tui_msv_layout

draw_multi_section_view previously inlined LayoutMetrics + bounds + measure + view.layout(...). Extracted to a pub fn tui_msv_layout(view, area) -> MultiSectionViewLayout that the rasteriser calls internally and tests/hosts call independently. This means:

• Paint and hit_test are guaranteed to use the same metrics (no drift possible from divergent inline definitions).
• Future MSV consumers in vimcode's TUI can call tui_msv_layout for click handling instead of building their own LayoutMetrics (eliminates the inline cell_quantum: 1.0 duplication that fix(quadraui): MSV snaps section bounds to integer cells (TUI cell_quantum) #297 had to add to mouse.rs and panels.rs).

Test plan

• cargo build --no-default-features clean
• cargo clippy --no-default-features -- -D warnings clean
• cargo fmt --check clean
• cargo test --no-default-features (1950 vimcode tests) green
• cd quadraui && cargo test --features tui (298 tests, +4 new harness) green
• Empirically verified: harness fails when cell_quantum=0.0, passes when cell_quantum=1.0

Files changed

• quadraui/src/tui/multi_section_view.rs — +337/-37. Extract tui_msv_layout; add 4 round-trip tests + helpers (tree_section, row_text, find_row_with, paint_then_layout).

Next steps (per PLAN.md "🧭 Course correction")

• Step 4: TreeView round-trip harness (same pattern, different primitive)
• Step 5: Extract quadraui to its own repo (0.0.1)
• Step 6: Re-do Migrate Debug sidebar to quadraui::MultiSectionView #296 with the harness gating
• Step 7: Migrate SC (quadraui: lift source control panel (inline commit input + split sections) #282) using the harness

🤖 Generated with Claude Code