refactor: focus model, bar update/render split, flicker & title fixes; +logging rotation, Lua REPL

[TheB1t]

Summary

Five commits on this branch, listed in git order (oldest → newest).

1. feat(log) — XDG state dir + rotation + per-call flush

• Log path via $XDG_STATE_HOME/sirenwm (fallback $HOME/.local/state/sirenwm), auto-created.
• Rotating file sink (5 MB × 5) plus stderr color sink.
• Split logs by app: sirenwm.log vs sirenwm-display.log (argv pre-scanned for --display-server).
• _LOG_EMIT flushes after every LOG_* so no lines are lost on crash.
• log_init_null() replaces /dev/null in tests.
• Wayland integration harness (run_tests_wayland.sh) now points at the new log location via a RUNTIME_LOG variable.

2. refactor(focus) — Core is sole producer of FocusChanged events

Earlier boundary fix: Core::dispatch(FocusWindow) emits FocusChanged only on actual state transition; the X11 arbiter becomes a mute applier. Removes 3–6× FocusChanged spam per real transition caused by three independent emit paths.

3. feat(debug_ui) — Lua REPL tab

• New REPL tab inside the existing debug window (no new window).
• Unrestricted eval/exec against the live Lua state via LuaHost::repl_eval.
• Expr-first fallback: tries return <code> first; falls back to statement block on load failure.
• Captures print() via thread-local buffer + repl_print; restores original print after the call.
• Debug UI only — no sandboxing.

4. refactor(focus) — single source of truth via Core::focus

Follow-up that collapses the focus subsystem onto a dwm-style single-writer model. Core::focus(WindowId) is now the only path that writes "who is focused". Removes FocusState cache, FocusPriority arbiter, ensure_focused, restore_visible_focus, request_focus, and the 60-line parent-walk in handle_focus_event. Intent chain: focused_monitor_ → Monitor.active_ws → Workspace::current.

Fixes:

• Borders not repainted after reload/restart.
• Double FocusChanged on cross-monitor EnterNotify.
• _NET_WM_STATE_FOCUSED sticking on the previously focused window when switching to an empty workspace — the else-branch in Core::focus() now emits FocusChanged(NO_WINDOW) unconditionally (symmetric with the focused branch), since prev read from wsman after a switch_to reflects the new workspace, not the window we are defocusing.

5. refactor(bar) — split widget update / render; restart/reload flicker & title fixes

• Widgets now separate expensive state mutation (update) from cheap rendering (render). update() runs on the interval schedule or on first load via prime_widgets(); render() runs on every repaint. Reactive widgets (interval=0) update+render on every redraw.
• Restart flicker: Core::dispatch(ApplyMonitorTopology) no longer emits DisplayTopologyChanged. Emission is the caller's responsibility — Runtime::dispatch_display_change() posts on hot-plug / reload; Runtime::start() does not (initial apply isn't a "change" and must not trigger reactive rebuilds that duplicate on_start).
• Reload flicker: BarModule::on_reload() no longer rebuilds physical windows — it only absorbs new config/theme. DisplayTopologyChanged (posted by dispatch_display_change() after reload) is now the single rebuild trigger.
• Missing focused-window title after restart: adopt path dropped title because ExistingWindowSnapshot didn't carry it. Title is now read directly from X at adopt time (not persisted across restarts) and threaded through snapshot → SetWindowMetadata. Single path through read_window_metadata; redundant read_window_title and local _NET_WM_NAME/UTF8_STRING interning removed.
• New LuaHost::call_ref_method_void for widget:update() (no return).

Test plan

• cmake --build build -j$(nproc) — clean.
• ctest --test-dir build --output-on-failure — 4/4 green (updated Hotplug.ApplyTopologyDoesNotEmitDomainEvent reflects the ApplyMonitorTopology contract change).
• CI Format (uncrustify) — green.
• X11 integration (run_tests.sh) — 05_focus now passes including the FOCUSED cleared after ws switch case.
• Wayland integration (run_tests_wayland.sh) — structured-log checks now resolve against the XDG runtime log path.
• Manual: bar no longer flickers on siren.restart() / siren.reload().
• Manual: focused-window title renders correctly on all monitors after restart.
• Manual: focus transitions produce exactly one FocusChanged per real change.
• Manual: logs rotate at 5 MB, split between sirenwm.log and sirenwm-display.log.
• Manual: REPL tab evaluates expressions, runs statements, captures print.

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