Replace hotplug polling with a registry watcher thread

[Magniquick]

Written with an LLM. Tested on real hardware, not just compiled.

Problem

The main loop chops every idle sleep into 10ms chunks only so poll_hotplug()
runs. Signals never needed it, since recv_timeout returns as soon as one
arrives. Each poll is a full Wayland roundtrip, so it wakes the compositor too.

Idle at night on my machine, the main thread averaged about 190 voluntary
context switches per second with nothing happening.

Fix

A watcher thread holds its own Wayland connection, binds only wl_registry,
and sends HotplugCheck when a wl_output global appears or disappears. The
main loop consumes that inside its sleep, so it does not re-evaluate or re-log
the schedule, and otherwise sleeps its whole interval in one wait.

Main-thread idle wakeups after: 0/s over 20s. The watcher is also 0/s, since it
blocks in blocking_dispatch. Process total goes from about 290/s to about
100/s; the remainder is the IPC server thread's 10ms recv_timeout in
src/state/ipc/server.rs, which this PR does not touch.

The watcher must not share the backend's connection. poll_hotplug() calls
EventQueue::roundtrip(), which blocks until its wl_display.sync callback
arrives, and a second reader can consume that callback and strand the roundtrip
forever. I hit this: the process hung and ignored SIGTERM.

HotplugMode replaces the implicit assumption that every backend polls. The
hyprsunset backend never overrode poll_hotplug, so it was waking once per
chunk to call an empty function. It now sleeps uninterrupted.

The loop still calls poll_hotplug() once per iteration before sleeping, which
covers what the watcher cannot: an output arriving while the backend is still
starting, and any HotplugCheck dropped while test mode or an interrupted
reload had the loop busy. If the watcher dies, its guard clears a liveness flag
and sends a final HotplugCheck, so the loop warns once and falls back to a 1s
poll instead of finishing a sleep that could run for hours unwatched.

Tested

Arch, Hyprland 0.56.2, Intel Xe.

• DRM CTM read back from the kernel with modetest, identical to the unpatched
  binary at 3300K/90% gamma. Four reloads tracked exactly; stopping leaves
  identity.
• Hyprland CTM backend, nested: hyprctl output create headless applies the CTM
  to the new output, remove drops it. Idle wakeups unchanged after both.
• wlr-gamma backend, nested headless sway: swaymsg create_output and
  output ... unplug both detected.
• Killing the compositor produces the fallback warning, and sunsetr keeps
  running.
• Clean SIGTERM in about 500ms. 243 tests, fmt and clippy clean.

Happy to split the HotplugMode change out if you would rather take it
separately.

[Copilot]

🔵 Needs a closer look

It introduces new cross-thread Wayland/event-loop behavior and alters main-loop sleep semantics in ways that are difficult to fully validate automatically.

Pull request overview

This PR removes frequent hotplug polling from the core sleep loop by introducing a dedicated Wayland registry watcher thread that notifies the main loop when wl_output globals appear/disappear, allowing uninterrupted sleeps while still reacting to output changes.

Changes:

• Added a HotplugMode backend capability to declare how output hotplug should be detected (NotNeeded vs WaylandRegistry).
• Introduced a Wayland-registry-only watcher thread that sends a SignalMessage::HotplugCheck to wake the main loop on output add/remove.
• Updated signal draining and test-mode handling to intentionally drop HotplugCheck to avoid re-emission/spin, with targeted unit tests.

File summaries

File	Description
src/io/signals.rs	Adds HotplugCheck and updates drain behavior + tests to drop it during reload draining.
src/core/tests.rs	Adds unit tests for registry watcher state tracking and liveness guard behavior.
src/core/mod.rs	Implements watcher thread + registry dispatch, integrates watcher-driven wakeups into the main loop sleep logic with fallback polling.
src/commands/test.rs	Ensures HotplugCheck is ignored during test mode to prevent spinning; adds a unit test.
src/backend/wayland/mod.rs	Declares Wayland backend uses HotplugMode::WaylandRegistry.
src/backend/mod.rs	Introduces HotplugMode and a hotplug_mode() trait method for backends.
src/backend/hyprland/mod.rs	Declares Hyprland backend uses HotplugMode::WaylandRegistry.

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 0
• Review effort level: Lite

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[psi4j]

I've got a whole lot going on right now with University and work, but I'll take a look at these as soon as I can allocate some free time to this, hopefully some time later this week.

[Magniquick]

I've got a whole lot going on right now with University and work, but I'll take a look at these as soon as I can allocate some free time to this, hopefully some time later this week.

same here :/ (hence the llms used). uni work is always a pita
best of luck :D