fix(quadraui): MSV snaps section bounds to integer cells (TUI cell_quantum)

[JDonaghy]

Summary

• LayoutMetrics gains a cell_quantum: f32 field. When > 0, MultiSectionView::layout snaps each section's resolved size to integer multiples of the quantum before emitting bounds. TUI passes 1.0; GTK passes 0.0 (sub-pixel Cairo).
• Fixes paint/click drift in TUI: paint already rounded bounds.y to integer rows, but hit_test consumed raw fractional bounds, so fractional EqualShare distributions (e.g. 4 sections in 21 cells → 5.25 each) made click at the row paint drew Watch's header on land in Variables' body. This is the bug class MultiSectionView was designed to eliminate; per-consumer Cell-on-engine bridge fields don't help — fix has to be structural inside the layout.
• Adds 2 regression tests in the primitive: integer-alignment of all section bounds, and paint↔hit_test agreement on every painted row.

Why this PR exists

Step 2 of the Session 346 course correction in PLAN.md. The full plan is harness-first quadraui work: revert #296 (already done — that work was on a feature branch never merged) → land cell_quantum standalone (this PR) → write TUI smoke harness for MSV → extract quadraui to its own repo → re-do #296 with the harness gating. This PR is independently valuable to Extensions (and any future MSV consumer) regardless of when #296 is re-attempted.

Distribution algorithm

Largest-remainder (Hare-Niemeyer): floor each fractional size, then award remaining cells to sections with the largest dropped fractional parts. The sum still equals usable_main exactly, so no off-by-one drift.

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 lib + integration tests green
• cd quadraui && cargo test — 217 tests green (215 + 2 new regression tests)
• Manual TUI smoke: open Extensions sidebar, scroll, click sections — bounds should be integer-aligned (no functional regression for current sizes; the worst-case fractional distribution mostly applied to the still-unmerged Migrate Debug sidebar to quadraui::MultiSectionView #296 Debug migration).

Files changed

• quadraui/src/primitives/multi_section_view.rs — cell_quantum field, integer-snap pass in layout_vertical, 2 regression tests (~165 lines)
• quadraui/src/tui/multi_section_view.rs — TUI metrics: cell_quantum: 1.0
• quadraui/src/gtk/multi_section_view.rs — GTK metrics: cell_quantum: 0.0 (explicit; matches Cairo sub-pixel paint)
• src/tui_main/mouse.rs, src/tui_main/panels.rs — vimcode TUI Extensions consumers updated to set cell_quantum: 1.0 on inline LayoutMetrics

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