Fix split-space admission after sidebar pane changes

[teamleaderleo]

Summary

Follow-up to #15392 that makes split admission use the actual prospective geometry and keeps pane construction side-effect free until admission succeeds.

• compute ratio-aware split-space verdicts with the real tree geometry
• preflight terminal, browser, simulator, Cloud, and sidebar splits before constructing their panels
• preserve the external-browser fallback when embedded browser creation is disabled
• remove the unreachable mixed-orientation fallback path

Evidence

• Added focused SplitSpaceTests and package-level SplitSpaceVerdictTests for prospective geometry, ratios, clamping, and rejection boundaries.
• git diff --check passes against current main.
• Independently reviewed at 705ed386baf8; no actionable findings.
• Structured review found no correctness or policy findings.
• No cmux app or native test build was run on Air Blue; hosted CI is the build/test authority.

Changelog

none

— Mochi

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Summary by cubic

Fixes split-space admission to use the real prospective geometry, so verdicts reflect the actual divider ratio and equalize behavior a split will produce. All split types now preflight for space before constructing panels, and pane construction stays side-effect free until admission succeeds.

• Split verdicts accept a divider position, clamp it to Bonsplit's 0.1...0.9 range, and simulate the exact equalized divider positions instead of averaging slots.
• Terminal, browser, simulator, Cloud, custom sidebar, and right-sidebar tool splits preflight before creating panels, terminals, or browser navigations; denied splits construct nothing.
• Room borrowing moved from split helpers into the Bonsplit delegate, now checking both the original and new pane extents.
• External-browser fallback still fires when embedded browser creation is disabled; session restore and remote-layout projection bypass admission explicitly, and the unreachable mixed-orientation fallback path is removed.

^{Written for commit 1764ca3. Summary will update on new commits.}

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between d2877b2 and 1764ca3.

📒 Files selected for processing (15)

• Packages/macOS/CmuxPanes/Sources/CmuxPanes/Geometry/ExternalTreeNode+SplitSpace.swift
• Packages/macOS/CmuxPanes/Tests/CmuxPanesTests/SplitSpaceVerdictTests.swift
• Sources/Surfaces/Workspace+CloudLayoutProjection.swift
• Sources/Surfaces/Workspace+CloudManualMirror.swift
• Sources/TabManager.swift
• Sources/TerminalController+ControlPaneContext.swift
• Sources/TerminalController+ControlSidebarContext3.swift
• Sources/TerminalController+ControlSurfaceContext2.swift
• Sources/Workspace+ClosedPanelLayoutRestore.swift
• Sources/Workspace+CustomSidebarPane.swift
• Sources/Workspace+RightSidebarToolDrop.swift
• Sources/Workspace+SimulatorPane.swift
• Sources/Workspace+SplitSpace.swift
• Sources/Workspace.swift
• cmuxTests/SplitSpaceTests.swift

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[github-actions]

Merge receipt for 1764ca3221: every check was green at merge (20 verified; 19 skipped by policy). Full suite runs on main after merge.