fix(worktree): show closing state instead of "Starting terminal…"

[neumie]

Problem

Closing a worktree left every pane painting the "Starting terminal…" placeholder for the whole teardown window — seconds on a real checkout — even though the terminals were already dead and the project was on its way out.

Root cause

prepare_background_worktree_removal (crates/okena-workspace/src/actions/worktree.rs:721) runs layout.clear_terminal_ids_except(&HashSet::new()): it nulls every layout leaf's terminal_id but deliberately keeps the project row and the tree shape standing, so ensure_terminal can't resurrect a PTY inside the doomed checkout.

The daemon then kills the PTYs (command_loop.rs:144) and only afterwards runs the blocking remove_dir_all; the project row is deleted much later (command_loop.rs:190). The client mirrors that intermediate snapshot verbatim, rebuilds each TerminalPane with terminal_id: None, create_new_terminal only logs a warning, and TerminalContent.terminal stays None → the uninitialized-slot placeholder at content.rs:605.

The id-nulling is correct and is not touched here.

Fix

• okena-layout: new allocation-free LayoutNode::has_terminal_ids() predicate.
• project_column.rs: the content branch becomes an explicit enum ColumnContent { Closing, Layout, Creating, Empty } picked by a pure column_content(), plus a render_closing_state placeholder mirroring the existing render_creating_state.

Closing has to win over Layout because a closing worktree keeps layout: Some — unlike is_creating, which sits in the else arm only because a mid-create project has layout: None.

Key call: Closing is gated on closing && !has_terminals, not on the closing flag alone. The merge and before_remove-hook phases also set is_closing while the terminals are still live and useful (and both are still abortable), so those keep painting the real pane tree. Only the id-less teardown window swaps to the placeholder.

Tests

• 5 unit tests over the branch table: nulled-ids-while-closing, live-terminals-while-closing, closing bookmark with no layout, optimistic client flag, and a guard that the non-closing branches are unchanged.
• 1 test in okena-layout pinning has_terminal_ids against clear_terminal_ids_except.

cargo build clean, clippy clean, 448 tests pass across okena-app / okena-layout / okena-workspace.

Notes

Based directly on #157 (refactorx/full-headless). This originally sat on top of #163; since that PR was closed unmerged, this branch was rebased with --onto to drop its three commits, so nothing from the closed PR rides along here.

Out of scope, spotted while tracing: command_loop.rs:237 (the JoinError arm of background removal) clears is_closing without the spawn_uninitialized_terminals restore its sibling git-failure arm performs. Panic-only path and recoverable in one click, so left alone here.

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https://claude.ai/code/session_01Y4Aq3X2s6ftafXzmqaN3A8

[matej21]

Integrated into #157 as commit d928be3 after review and combined validation.