Add DockEvent stream + show_inside_with_response

[enomado]

Motivation

Currently DockArea::show_inside returns (), so a host that wants to push an undo entry on layout changes has to diff the serialised state every frame. That diff is non-zero on every frame the user holds and drags a separator, which produces one undo entry per frame instead of one per drag.

What this PR adds

• DockArea::show_inside_with_response(ui, viewer) -> DockAreaResponse. The existing show_inside is kept as a thin wrapper — no behaviour change for current callers.
• DockAreaResponse { events: Vec<DockEvent> } with helpers:
  • layout_changed() — any mutation this frame, including in-progress drag.
  • layout_committed() — finalised mutations only; right trigger for undo / disk save.
• DockEvent enum with two variants today:
  • SeparatorDragging — emitted per frame while the user holds and drags a separator (continuous).
  • LayoutCommitted — finalised: tab close / move / detach, leaf collapse, window minimise, separator drag end / arrow-key nudge / double-click reset, focus change.

Both DockEvent and DockAreaResponse are #[non_exhaustive] so future fine-grained variants (SeparatorDragCommitted { before, after }, TabClosed(TabPath), …) can land without breaking consumers that go through layout_committed().

Drive-by fixes

Two pre-existing bugs surfaced while wiring events in. Clicking an already-active tab title and OnCloseResponse::Focus on the already-active tab were writing leaf.active = tab_index unconditionally — minor on its own but breaks the new event invariant ("LayoutCommitted ⇒ real mutation"). Both now guarded with if leaf.active != tab_index.

Compatibility

Strictly additive on the public API: show_inside keeps the same signature, DockEvent and DockAreaResponse are new types. No semver bump needed.

Test plan

• cargo test --all-features — 20 passed.
• cargo build --all-features — clean (only pre-existing warnings).
• cargo clippy --all-features — no new lints from this PR.
• Real-world consumer (the project that motivated this) confirms separator drag now produces one undo entry per drag, not per frame.

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