feat(split): two tabs side by side on top of herdr's own layout - #861

#3072)

Part of #1158.

## What

The split view can hold up to four panes in rows and columns, behind a
new per-device setting, **Settings > Terminal > Split into rows and
columns** (`splitGrid`, off by default). With the setting off, the split
view keeps the two halves from #861/#1176, so nothing changes until a
device turns it on.

With it on:
- A sidebar row, an Agents row, a tab or a slot's bar dropped on a
slot's **edge band** (a quarter of its width or height) opens the pane
in a new slot beside, above or below that slot. Dropped on the
**middle**, it opens in that slot's place. A single pane keeps the
halves' midpoint with its 12px band, plus a band along its top and
bottom.
- The layout is a binary tree of row and column splits with **at most
four slots**. Every slot keeps a one-letter id and is drawn in one
container at the rectangle the tree gives it, so a slot **keeps its
terminal and its connections** when the layout around it changes.
- Each **boundary** is a `role="separator"` that can be dragged, or
focused and moved with the arrow keys (5% steps, between 15% and 85%).
- The guide outlines every slot and moves one accent highlight to the
place the pane would take ("Drop above", "Drop below", "Open here",
...).
- Closing the active slot gives its place and the selection to the slot
it shared a split with. Two slots side by side are the two halves again,
and closing one of those is "Close this half" as before.
- The layout is kept in `ovis:split` across a reload. A split saved by
the two-half version opens as before.

Shared with the halves: a pane may show in several slots. The active
slot sizes its pty when it shows the tab, else the first slot in reading
order does, and the others follow (PaneFollowContext). Alerts, following
herdr's focus and a pane moved under a new ID (#1503) now look at every
other slot, not just the one other half.

## Scope

#1158 asks for three things. This PR ships the first and most-requested
one: three or more views in one window, resizable, with the existing
drag and drop (vibewriter-hand's comment asks for exactly this first).
Moving a pane to a window of its own, Option+Tab between slots, and
windows knowing each other over BroadcastChannel are left for
follow-ups.

The tree model (`slotsOf`, `slotRects`, `splitDividers`, `insertBeside`,
`pruneSlots`, `dropZone`, `zoneRect`, the old-format read in
`storedSplit`) follows bricks-8's fork (`feat/window-presence`, first
commit), credited as co-author. Their branch predates #1176, which lets
one tab show in both halves, so the drop rules here follow #1176 (a drop
never moves or swaps another slot's pane) instead of the fork's
swap-on-drop. The limit is four slots instead of six, as the issue's
follow-up comment asks for "three or four" first, and each slot past `a`
opens another set of connections.

Design reference: VS Code's editor groups (split right or down into a
grid, draggable boundaries, the drop overlay by edge or centre). The
DESIGN.md entry says so.

The chat composer gets no new control. The only new UI is the opt-in
Settings toggle, plus the boundaries and guide that appear only with it
on.

## Screenshots (demo build, setting on)

Taken in the CI runner image from the demo fixture. Light and dark at
1280 (three panes, four panes with a focused boundary, the bottom-edge
and left-edge guides), and the Settings row at 1280 and 390. The images
are attached in the lane report. The phone width never draws a split (up
to 768px shows the active slot alone, as before).

## Validation

- `bun test src/lib/split.test.ts`: 26 pass. New cases cover top/bottom
bands, a third pane below a slot, the four-slot cap, closing into the
sibling's place, picks in another slot, boundary limits, and reading a
two-half save and a grid.
- `bun test src/lib/settings.test.ts src/lib/followFocus.test.ts
src/lib/paneMove.test.ts`: pass.
- `scripts/split-view-regression.ts` (checkSplitView, plus
checkTabOverflow, which also uses the split) in the CI runner image
(`herdr-ci-runner:local`): all PASS, including the new case "rows and
columns: a pane opens below a slot, slots keep their terminals, a
boundary moves from the keyboard and a reload keeps the layout". The
existing two-half cases run unchanged with the setting off.
- `bun run typecheck`: clean.
- `bun run check fast`: see the lane report for the result at the PR
head.
- Skipped: full `check full` and the other browser lanes, which CI runs.

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Co-authored-by: bricks-8 <225523807+bricks-8@users.noreply.github.com>