Fix right-panel split removal and drop bounds

[brsbl]

Summary

• Keep Info and Diff fixed.
• Add a trailing pane-edge Remove split button only while the right panel has multiple panes.
• Rehome every tab into the surviving pane without invoking the tab close callbacks.
• Confine tab-split drag targets to the right panel; the web workspace no longer accepts right-panel tab drops.

Product decision

Info and Diff remain fixed. Making them closeable would change Cmd+W, Cmd+D, mobile, and persisted-layout behavior. This layer instead preserves the existing two-action model: the tab-pill × closes a closeable tab, while the pane-edge × removes a split without closing its tabs.

Before

The fixed Info and Diff panes had no remove-split affordance.

After

Each split pane exposes a keyboard-focusable, trailing Remove split button; no control is present after the layout returns to one pane.

Validation

• Regression coverage: split-only visibility, accessible naming, native-button Enter/Space activation, tab preservation, focus/selection semantics, tab-close independence, and in-panel/out-of-panel drag targeting.
• Real web flow: mouse and keyboard split creation/removal, closeable tab close behavior, right-panel tab reorder, right-panel split drop, and rejected main-content drop.
• Real Safari 26.5.2 flow: keyboard-created fixed Info/Diff split, two focusable Remove split buttons, Enter removal, and both fixed tabs preserved after unsplitting.
• pnpm exec turbo run test --filter=@bb/app --force -- --run ... — 9 files, 159 tests passed across the stack.
• pnpm exec turbo run typecheck --filter=@bb/app — passed.
• pnpm exec turbo run lint --filter=@bb/app — 0 errors; existing warnings only.

Closes #2450

BB-Thread-ID: thr_q8degf2y66

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