Tasks: Latest-turn changes

• T001 Server integration tests first (crates/signaltty-server/tests/turn_diff.rs): capture on working, blocked continuation, committed + uncommitted turn work, earlier dirt excluded, turn: null, turn file read without a turn.
• T002 turns.rs: baseline registry, private-index snapshot, capture task on agent.working, workspace.turn_started.
• T003 workspace.diff / workspace.file_diff accept scope; tree-to-tree summary and file read; workspace.close forgets the baseline.
• T004 CLI workspace diff --turn (extends cli_workspace_diff_reports_counts).
• T005 GUI All / Turn toggle, scope wording, turn refreshes; headless tests plus GTK test changes_panel_scopes_to_the_latest_turn with light/dark captures.
• T006 ADR-0029, docs/08-ipc.md rows and event, ADR-0011 status.