feat: add neighbor peeks to scrolling layout

[tingkai-c]

Summary

Adds opt-in neighbor peeks to the scrolling layout's Niri navigation mode.

When enabled, the immediate columns beside the focused column remain partially visible. With focus-follows-mouse enabled, hovering a visible neighboring sliver focuses that column without allowing focus to cascade repeatedly across the strip.

Configuration

[settings.layout.scrolling]
focus_navigation_style = "niri"
neighbor_peek_width = 24
neighbor_peek_rearm_distance = 20

neighbor_peek_width defaults to 0, preserving existing behavior.

neighbor_peek_rearm_distance defaults to 20 pixels and controls how far the pointer must move horizontally before another neighbor-peek focus change can occur.

Behavior

• Keeps immediate neighboring columns partially visible in Niri mode.
• Prioritizes the navigation-side neighbor when both peeks cannot fit.
• Supports focus-follows-mouse interaction with the exposed slivers.
• Latches after a hover-triggered focus change to prevent accidental focus cascades.
• Re-arms after the configured horizontal pointer movement.
• Uses generation tracking so stale asynchronous confirmations cannot affect a newer hover.
• Resets hover state after clicks, drags, Space changes, layout-mode changes, and configuration reloads.
• Adds semantic validation for both configuration values; Rift's existing hot-reload path enforces it.

Compatibility

The feature is opt-in. Existing scrolling behavior remains unchanged unless neighbor_peek_width is set to a positive value.

Testing

Added 12 focused tests across layout, input handling, reactor behavior, and configuration.

• Layout tests cover bilateral peeks, direction-prioritized peeks, neutral reveals, and column adjacency.
• Input tests cover hover latching, re-arming, mode gating, and stale generations.
• Reactor tests cover clipped-window detection and cascade suppression.
• Configuration tests cover defaults and validation.

Full test suite: 420 tests passed.

Demo

neighbor-peek-demo.mov

[tingkai-c]

Thanks for merging main into this branch. The event-tap conflict resolution dropped the upstream tap_recovery_discards_cached_keys_and_uses_live_flags test, so I restored it unchanged in 597f697. I also ran the full test suite on the merged branch: 440 tests passed.

[tingkai-c]

Rebased onto current main and resolved the conflict. I also fixed a hover re-arming edge case found during review and added regression coverage. All 484 tests and nightly rustfmt pass; the PR is mergeable again.