Fix right-panel split removal and drop bounds

[brsbl]

Summary

• Keep Info and Diff fixed rather than making them generally closeable.
• Add a trailing, keyboard-focusable Remove split × only while a right-panel tab is in a split.
• Preserve the tab-pill × as the close action for normally closeable tabs; the pane-edge × removes the split without closing its tabs.
• Confine tab-split drop targets to the right panel while preserving right-panel split creation and tab reordering.

Product decision

Info and Diff remain fixed. Making them closeable would change existing Cmd+W, Cmd+D, mobile, and persisted-layout behavior. This layer extends the existing thread/page split semantics instead: closing a tab and removing a split remain two distinct actions.

Screenshot evidence

All web captures use Chrome for Testing 151.0.7922.71 at the same 1440×900 CSS viewport (DPR 2) and the same one-file Split tab UX verification fixture. The before revision is the PR merge base b629714e31487bde8af5574639422a9b5c97cbb5; the after revision is exact PR head 35050edb632c1007f14978c6c5e5bfe4dd83a259.

Split-only remove control

Before, fixed Info and Diff panes have no remove-split affordance.

After, each split pane has a trailing ×. Its accessible name is Remove split; it disappears when the layout returns to one pane.

Drag boundary

Before, dragging a right-panel tab over the main workspace exposes the web-only Group tab here target.

After, the same drag over the main workspace has no drop target.

The right panel still exposes its intended split target.

Validation

• Capture manifests record the exact revisions above, identical fixture and viewport, zero rendered error states, two Remove split controls after the change, no main-workspace overlay after the change, and a preserved Split bottom target inside the right panel.
• Regression coverage in this layer targets split-only visibility, accessible naming, native-button Enter/Space activation, tab preservation, close-action independence, and in-panel/out-of-panel drag targeting.
• Tests, typechecks, lint, and other CI-equivalent checks run on the pull request through GitHub Actions; none were run locally for this screenshot audit.

Closes #2450

BB-Thread-ID: thr_q8degf2y66

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