Add last focus behaviour for navigation between panes

[max-legrand]

Added last_focus behavior for navigation (split out from #60)

https://ampcode.com/threads/T-019b95f7-cb4d-70d9-9bf1-a6785cf358ee

[github-actions]

Review Summary

The remember_focus navigation feature is well-designed overall with good documentation and proper serialization support. Found two issues to address:

1. Bug (line 2185): Duplicate call to update_pty_focus after set_focus_and_track which already calls it internally.

2. Consistency issue (line 1399): move_focus manually updates focus state and only tracks the immediate parent split, while set_focus_and_track properly walks the entire ancestor path. This could cause last_focused_child_idx to be stale in grandparent splits when using keyboard navigation vs. mouse clicks.

The rest of the implementation looks solid - the type definitions, get_preferred_leaf, find_node_in_tree, and serialization changes are all correct.

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View full review context

src/lua/tiling.lua:1399: Inconsistent focus tracking

This manually updates state.focused_id and only tracks the immediate parent split (lines 1404-1407), while elsewhere set_focus_and_track is used to properly track all ancestor splits in the path. This could lead to inconsistent last_focused_child_idx state in deeply nested splits.

Consider using set_focus_and_track(target_leaf.id) here instead of the manual approach, then just call prise.request_frame() after.

[github-actions]

Bug: Duplicate update_pty_focus call

set_focus_and_track already calls update_pty_focus(old_id, state.focused_id) internally (line 846). This second call is redundant and could cause unnecessary PTY operations.

Suggested change

	update_pty_focus(old_focused_id, state.focused_id)
	set_focus_and_track(new_pane.id)
	state.pending_split = nil
	prise.request_frame()
	prise.save() -- Auto-save on pane added
	return