fix(events): background interactive hooks to prevent tmux freeze

[rtalexk]

Context

Interactive hooks fire on user interactions (mouse scroll, window/session switch).
Currently, these hooks use synchronous run-shell commands, which block the tmux
server's command queue while demux event performs database work. When DB
operations stall on the SQLite lock (held by state-writer hooks), the tmux
server freezes mid-redraw, tearing the visual paint and breaking copy-mode
selection rendering.

This fix backgrounds the interactive hooks to allow tmux rendering to continue
while event handlers run concurrently, preventing the visual freeze.

What does this PR do

• Adds -b flag to pane_focus, window_focus, and session_changed hooks to
  background their execution, preventing the tmux server from blocking on DB work.
• Wraps pane_focus event handler logic in withEventLock to serialize
  concurrent invocations (since the hooks are now backgrounded) and prevent race
  conditions in sidebar repositioning.
• Adds TestInteractiveHooksAreBackgrounded to verify that all four interactive
  hooks (after-select-pane, client-focus-in, after-select-window,
  client-session-changed) have the -b flag set.
• Adds debug logging for pane_focus resolution and event processing.

Out of scope

None. All changes are contained to the event hook system.

Manual QA

Verify that interactive events (pane focus, window/session switch) no longer
freeze the tmux server's redraw cycle.

Prerequisites

1. demux is built and in PATH

demux --version

Expected: output shows version (e.g., demux dev).

2. Hooks are registered in the test tmux server

# Create a test session
tmux new-session -d -s test-demux -c ~

# Verify hooks are set
tmux show-hooks -t test-demux

Expected: output includes after-select-pane, after-select-window,
client-focus-in with -b flag in their run-shell commands.

Setup

1. Build and install the binary:

   go build -o demux ./cmd/demux
   sudo mv demux /usr/local/bin/

2. Create a fresh test session:

   tmux new-session -d -s test-render -c ~ -x 120 -y 40
   demux attach test-render

3. Capture baseline: open several panes and note that window/pane switches render smoothly.

Scenario 1 — Rapid pane switches don't freeze render

Rapidly switch between panes in a multi-pane window

1. In the test session, split into 3+ panes:

   tmux split-window -t test-render -h
   tmux split-window -t test-render -v

2. Rapidly switch focus between panes (hold Alt+arrow or use mouse click):
   • Click pane 1 → pane 2 → pane 3 repeatedly for ~10 switches.
3. Expected: No visual glitch, no torn paint, no delay in redraw. The cursor
   follows focus cleanly.
4. Check debug logs (if enabled):

   tail -f ~/.cache/demux/demux.log | grep "pane_focus"

   Expected: log lines appear with no errors.

Scenario 2 — Rapid window switches don't freeze render

Create multiple windows and switch between them rapidly

1. Create additional windows:

   tmux new-window -t test-render
   tmux new-window -t test-render

2. Switch windows rapidly (Ctrl+N, Ctrl+P, or mouse click on window list):
   • Switch between windows 10+ times over 5 seconds.
3. Expected: No visual freeze, smooth render, sidebar (if visible) repositions
   without lag or paint tears.
4. Verify event processing in logs:

   tail -f ~/.cache/demux/demux.log | grep "window_focus"

   Expected: events logged with no errors.

Scenario 3 — Test suite passes

Run the test for hook backgrounding

cd /Users/rtalex/com.github/demux/main
go test ./internal/tmuxhooks -run TestInteractiveHooksAreBackgrounded -v

Expected: test passes, confirming all four interactive hooks have -b set.

Run all tmux hook tests:

go test ./internal/tmuxhooks -v

Expected: all tests pass.