feat(app): add unfocused split-pane dimming — configurable opacity for visual hierarchy

[griffinwork40]

Unfocused split-pane dimming

When a split is active, the inactive pane dims to a configurable alpha value — making it clear at a glance which pane has focus without hiding the adjacent context.

Config

"unfocusedPaneOpacity": 0.85

0.0–1.0. Default 1.0 = no dimming (no visual change for users who don't set this). 0.85 is a good starting point. Out-of-range values are rejected with a stderr warning and the default is kept.

Hit ⌘R after editing ~/.config/umber/config.json to see the change live.

What changes

File	What
Config.swift	ConfigFile.unfocusedPaneOpacity (optional Double) + AppConfig.unfocusedPaneOpacity (Double, default 1.0) + fail-soft load path
SplitContainerView.swift	setFocusedChild(_:opacity:) — sets alphaValue on both child views with a 150ms ease-in-out animation (skipped at 1.0 so the common case is zero-overhead). didReceiveClickInChild callback + hitChild(at:) so mouse clicks update dimming before firstResponder changes
SpaceViewController+Splits.swift	updateSplitDimming(for:) resolves the focused child via the isDescendant first-responder walk (same path as focusedShellHost) and drives the container. Called from: split creation, ⌘⇧H/J/K/L keyboard navigation, config reload, and mouse clicks. Dimming + callback are cleared at every teardown path
SpaceViewController.swift	One line in apply(config:) to refresh active-split dimming after ⌘R
StarterConfig.swift	Documents the new field with a usage example

Behaviour

• Single-pane terminal: always full opacity — feature is invisible unless you're in a split.
• Split creation (⌘⇧\): primary has focus, peer dims immediately.
• Keyboard focus (⌘⇧H/J/K/L): 150ms animated transition as focus crosses.
• Mouse click on the unfocused pane: dimming flips before firstResponder changes, so the response feels instantaneous.
• Config reload (⌘R): new opacity takes effect on the active split immediately.
• Split close / peer exit / tab close: alpha is reset to 1.0 and the callback is cleared before the view is released.
• Both engines: alphaValue is set on the documentView (the view the container owns), so it works identically with SwiftTerm and GhosttyKit panes without naming either engine.

File sizes

All files within the 350-LOC ceiling. Largest touched file: SpaceViewController.swift at 348 (was 344 before; 2 lines from the ceiling, same as before this PR).

Config.swift                  336  ⚠ within 14
SpaceViewController.swift     348  ⚠ within 2
SpaceViewController+Splits.swift  272
SplitContainerView.swift      323  ⚠ within 27
StarterConfig.swift            63

Concerns

• SpaceViewController.swift is at 348/350. The one line added to apply(config:) was necessary (it's the only fan-out path for config reload), and it's a comment + one line of code. The file should be split before anything else lands there.
• Mouse-click dimming depends on SplitContainerView.mouseDown, which fires only when the event reaches the container — if an engine-internal view intercepts mouseDown before it bubbles to the container, the click callback won't fire. In that case dimming still corrects on the next ⌘⇧H/J/K/L or config reload. This is acceptable for v1 given both engines let events bubble normally through NSView.
• No gate added. Dimming is an AppKit visual effect that imports NSAnimationContext and CAMediaTimingFunction — both ungatable in the headless swiftc harness pattern. Covered by daily-drive and the file-size gate.