Add LUI split: Bonsplit-style tabbed split panes for all platforms

[tiensonqin]

Summary

Adds a Bonsplit-style tabbed split-pane library (Lui_split) covering every lui backend: SwiftUI, Qt/QML, Flutter, WinUI, and Web — with drag-and-drop tab reordering, edge-drop splits, divider resizing, keyboard navigation, and platform-owned gesture animations (120fps: drag visuals never cross the bridge; only committed actions become events).

Architecture: four extension components — split-view > split-branch > split-pane > split-tab — declared once in src/lui_split.ml and rendered natively per backend via the existing Lui_extension mechanism (profiles gate them off GenericOS/GenericHost). Lui_split.Model is the Bonsplit BonsplitController analog: a pure OCaml t + action reducer (update) + render that emits the extension tree and wires pane_action decoders into a dispatch callback — identical split policy on every platform. Apps embed Model.t and dispatch actions:

| SplitPanes action ->
  { model with split_panes = Lui_split.Model.update model.split_panes action }

(* view *)
Lui_split.Model.render
  ~build:(fun tab -> [ my_tab_content tab ])
  ~dispatch:(fun a -> send (Model.SplitPanes a))
  model.split_panes

Events: tab-selected, tab-closed, tab-moved{tab,index,from-pane}, split-drop{tab,from-pane,edge} (edge drop → new pane at 0.25 share, matching Bonsplit's edgeRatio), navigate{direction}, split-requested, pane-focused, pane-closed, ratio-changed.

Per platform:

• Apple (LUISplit.swift): LUISplit.register(in:), LUISplitSettings via EnvironmentValues, ⌘⌥D/⌘\/⌘W/⌘⇧W/⌘⌥arrows shortcuts, DragSource/edge drop zones.
• Qt (5 QML files + LUI::registerSplitExtensions): Loader-mounted components, Drag/DropArea with lui-split-tab keys, Ctrl/Alt shortcuts.
• Flutter (lui_flutter_split.dart): LUIFlutterSplit.register, Draggable/DragTarget chips (global→local offset via GlobalKeys), CallbackShortcuts, Offstage+TickerMode per-tab content.
• WinUI (LUISplitExtensions.cs): SplitView/Branch/Pane/TabControl visuals, CanDrag/AllowDrop, modifier-key shortcuts via InputKeyboardSource. Not compile-verified locally (no dotnet on this machine) — CI covers LUI.WinUI.
• Web (lui-split.js/css + demo/lui-split.html): standalone renderer + LUISplit.mount demo host applying the same semantics locally.

Also: generator gains qml/winui hosts and annotates decoded event records (fixes ambiguity when two events share a field shape); gallery registers the split components and adds a Model-driven demo section (Apple-gated, like apple-map).

Tests: fingerprint literals are asserted in sync across all six host sources (Swift/C++/Dart/C#/JS/OCaml), plus extension op round-trips, event decoders, and reducer coverage (move/index adjustment, edge-drop geometry, navigation, pane collapse, last-pane protection). Verified: dune runtest 28 pass, swift build, cmake + tst_lui_qml_backend 14/14, flutter analyze clean, node --check + schema tests pass.

Link to Devin session: https://app.devin.ai/sessions/becd167aced74606b789306295fef578
Open in Devin Desktop: https://app.devin.ai/desktop/session/becd167aced74606b789306295fef578?variant=devin
Requested by: @tiensonqin

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[devin-ai-integration]

I'll fix CI failures and address comments from users with write access. I'll skip comments containing "(aside)".

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[devin-ai-integration]

✅ Devin resolved all 9 findings on a7f1586

Fixed by Devin (9)

• Gallery rejects its new split section
• Stale drop loses a tab
• Protected tabs close by keyboard
• Flutter divider forgets prior drag motion
• Qt divider drag oscillates
• Web demo ignores tab moves and splits
• Qt zero split ratio becomes half
• Web demo reorders tabs one slot late
• Untrusted tab content executes as HTML

View all findings in Devin Review

[devin-ai-integration]

E2E-tested on both demo targets — the standalone web host (platform/web/demo/lui-split.html) and the native LUISplitDemo.app (SwiftUI/AppKit + real OCaml model). Several bugs were found and fixed along the way (emit-redraw eating clicks, paneOf resolving the parent instead of the pane, divider snap-back, macOS ⌥ character rewriting, .function modifier on arrows, × swallowed by chip gestures).

Web demo — fully green: click-select, × close, cross-pane tab move, edge-drop split at ~25% share, divider drag persists, in-strip reorder, Ctrl+Alt+D split, Ctrl+Alt+arrows navigate (focus follows the model), Ctrl+W close + empty-pane prune.

Native app — green except one OS limitation: click select, × close, ⌘⌥arrows, cross-pane move, edge-drop ~25%, divider, ⌘\ split, ⌘W close+collapse all pass. ⌘⌥D is consumed by macOS for Dock autohide before it reaches the app — verified the event never arrives. ⌘\ is the working split shortcut.

[devin-ai-integration]

Re-verified after the slide-in animation (6c740f7) — fresh panes spring open on both edge-drop and ⌘\ splits while the first render stays static. That pass surfaced three more bugs, all fixed in 2b1bd22:

• Center-pane drops silently no-op'd — the drop delegate sent index: Int.max, which overflows the OCaml bridge's JSON-int guard (2^62), decodes as a float, and fails the tab-moved decoder. Now sends the real append index.
• Cross-pane chip drops — same silent-drop path via the pane delegate; now insert at the indicator position.
• ⌘⌥arrows only moved the focus border — the focused property now drives FocusState, so nav → ⌘W closes the navigated pane's tab.

Final golden-path video: launch (static first render) → select → × close → cross-pane move → edge-drop slide-in → divider → ⌘\ slide-in → ⌘⌥nav → ⌘W.