Add app.equalizeSplitsOnCreate to balance panes on new splits

[teamleaderleo]

Fixes #731

Adds app.equalizeSplitsOnCreate (default false, so nothing changes unless you turn it on). When on, creating a split rebalances the panes in that split's row or column, so repeated Cmd+D gives 50/50, then thirds, then quarters instead of 50/25/12.5.

How it works:

• One shared step, Workspace.equalizeSplitsAfterCreatingSplitIfEnabled, runs the new PaneLayoutService.equalizeSplitRun. It equalizes only the run of same-orientation splits that holds the new pane (a pure plan in CmuxPanes, equalizeDividerPlan(forSplitRunContainingPaneId:), with the same span weighting as Equalize Splits). A vertical split never moves the dividers between side-by-side panes, and a split in one row leaves hand-tuned dividers in other rows alone.
• It runs after user-initiated splits: TabManager.createSplitOutcome (keyboard shortcut, menu, context menu, command palette, Ghostty split action passthrough), TabManager.newSplit (AppleScript), TabManager.newBrowserSplit (browser split actions), the v1 new_split and v2 surface.split / pane.create socket commands, and bonsplit's own split buttons.
• An explicit divider position (for example --ratio on the CLI) still wins. Session restore, workspace layouts, remote tmux mirrors and canvas mode are left alone. The Dock split store, Cmd-click links that open in a new split, and drag-to-split are not covered.

Setting surfaces: settings catalog, cmux.json path support and template, schema (embedded copy regenerated), skills/cmux-settings/references/all-keys.md, and a toggle in Settings > App next to the focus history row, localized in all nine macOS locales.

Tests: TabManagerEqualizeSplitsOnCreateTests covers off (50/25/25), on (thirds), orientation scoping, unrelated rows staying put, and an explicit divider winning. SplitGeometryTests covers the run plan. SettingCatalogTests checks the default is off.

Known limit: when the run spans more than the new split, the other panes' terminals keep their old frames until their anchors re-layout, the same as running Equalize Splits by hand. The new split's pair still gets its provisional frames.

Not duplicating #1701 or #3200: those add an equalize shortcut, which main already has. This one equalizes automatically on create.

Thanks @aibakun for the request.

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between 8538fa9 and d3b1366.

⛔ Files ignored due to path filters (1)

• Packages/macOS/CmuxFoundation/Sources/CmuxFoundation/ConfigValidation/CmuxConfigSchema.generated.swift is excluded by !**/*.generated.*

📒 Files selected for processing (22)

• Packages/macOS/CmuxPanes/Sources/CmuxPanes/Geometry/ExternalTreeNode+SplitGeometry.swift
• Packages/macOS/CmuxPanes/Sources/CmuxPanes/PaneLayoutService.swift
• Packages/macOS/CmuxPanes/Tests/CmuxPanesTests/SplitGeometryTests.swift
• Packages/macOS/CmuxSettings/Sources/CmuxSettings/Keys/AppCatalogSection.swift
• Packages/macOS/CmuxSettings/Tests/CmuxSettingsTests/SettingCatalogTests.swift
• Packages/macOS/CmuxSettingsUI/Sources/CmuxSettingsUI/Navigation/CuratedSettingEntry+Default.swift
• Packages/macOS/CmuxSettingsUI/Sources/CmuxSettingsUI/Sections/AppSection.swift
• Packages/macOS/CmuxSettingsUI/Tests/CmuxSettingsUITests/SettingsRowAnchorResolutionTests.swift
• Resources/Localizable.xcstrings
• Sources/CmuxSettingsFileStore+SupportedPaths.swift
• Sources/CmuxSettingsJSONPathSupport.swift
• Sources/KeyboardShortcutSettingsFileStore+Template.swift
• Sources/TabManager+TerminalSplitAction.swift
• Sources/TabManager.swift
• Sources/TerminalController+ControlPaneContext.swift
• Sources/TerminalController+ControlSurfaceContext2.swift
• Sources/TerminalController.swift
• Sources/Workspace+EqualizeSplitsSupport.swift
• Sources/Workspace.swift
• cmuxTests/TabManagerUnitTests.swift
• skills/cmux-settings/references/all-keys.md
• web/data/cmux.schema.json

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Merge receipt for d3b13669f1, merged 2026-09-25 20:11:13 UTC

• Not verified at merge: ci-status (not reported), browser-skill (in progress), detect-ios-changes (in progress), Fast static checks (in progress), Web complexity candidate (in progress)
• Verified: Web complexity, web-validation, catalog-structure, runner, Testbox broker trust boundary
• Skipped by policy: web-build, web-database-tests, web-tests
• Full suite: runs on main after merge.

Labeled merged-unverified: if main breaks near this merge, look here first.