feat: explode mode layout control — repack, minimize, swap

[mthorme]

Explode mode gave no control over its grid. Three changes plus two fixes found while using it.

Repack — no dead cells

The old repeat(cols, 1fr) x repeat(rows, 1fr) grid wasted space on uneven counts: 3 terminals in 2 columns put the third at half height beside an empty cell. Cells now get explicit pixel rects, packed column-major so the odd one out owns a column at full height.

Rects rather than grid tracks because the cells are one flat tabs.map shared with normal mode. Wrapping them in per-column elements to express this would change the React tree between modes and remount every terminal, losing xterm scrollback on each toggle.

Minimize — to a header tray

A minimized terminal leaves the grid entirely and parks beside the "N tasks" label (explode is the one layout with spare header room). Survivors repack over it, so minimizing actually buys space rather than leaving a collapsed strip still holding a row.

It stays mounted and inert, not unmounted — it keeps running with its scrollback, which is the point of minimizing rather than closing. Focus is never left on a minimized cell, or shortcuts would route to a terminal the user cannot see.

Swap — drag one cell onto another

A hover-revealed grip exchanges exactly two positions. Swap, not splice: a splice cascade-shifts the tail, reading as "everything moved" when one trade was asked for.

Native DnD with a custom mime rather than @dnd-kit (which is available): the layout is custom rects, not a DOM-ordered sortable list, and the mime keeps file/text drags falling through to the terminal that should handle them.

Two fixes found in use

Drops were swallowed over terminals. Terminal.tsx attaches a dragover listener calling stopPropagation() unconditionally (it owns file-drop-to-paste-path), so a bubbling cell handler never fired over terminal content. Whether a swap worked came down to landing on non-terminal chrome — which correlates with cell size, so full-height cells appeared un-droppable. Handlers moved to the grid in the capture phase.

Sub-pixel churn. Fractional rects let xterm's row count oscillate between N and N+1 on observer jitter, each flip a real fit(), SIGWINCH and atlas re-raster — ensureFit is idempotent on a stable geometry, but an oscillating one defeats it. Rects snap to whole pixels, rounding edges so neighbours still tile exactly. The ResizeObserver also re-rendered the whole app tree on sub-pixel jitter; now rounded with a bail-out.

Verification

• explodeLayout.test.ts — 65 checks: packing, the 3-in-2-columns regression, exact tiling, whole-pixel rects at awkward sizes (1001x601), swap semantics, order reconciliation
• Typecheck + biome + lint:theme clean
• Exercised in a live dev build

Known / deliberate

• Arrangement is session-scoped — minimized state and custom order reset on app restart. Persisting to the tab store is a natural follow-up, left out rather than guessing where it belongs.
• xterm's scrollbar can look slightly off — cell heights are whole pixels but not multiples of the terminal row height, so a few pixels of remainder sit below the last row while the scrollbar spans the container. Pre-existing (old fractional track heights were worse); a real fix means snapping to row multiples, which the engine cannot do without knowing each cell's non-terminal chrome.