Fail loudly when a focus backend's enumeration API is missing

[jonocodes]

Context

The GNOME focus extension's ListWindows() guards the enumeration call with a ternary:

const actors = typeof global.get_window_actors === "function"
  ? global.get_window_actors()
  : (typeof global.display.get_window_actors === "function"
      ? global.display.get_window_actors()
      : []);

The final : [] fallback is exactly what hid the stage-2 (#126) bug fixed in e166242: the code called the method on the wrong receiver (global.display instead of global), the guard swallowed the missing method, and ListWindows silently returned [] on every real GNOME 50 session. The running-windows list showed "no running programs" and looked like an honest empty state rather than a broken API call.

Ask

Decide whether a missing enumeration API should fail loudly instead of degrading to an empty list. Options:

• Keep the empty-list fallback but log() a one-time warning when neither global.get_window_actors nor global.display.get_window_actors is a function, so the journal shows the API drift.
• Or drop the silent fallback entirely and let the method throw (the daemon's watch_windows already logs gdbus failures at debug and sleeps through them — a thrown error there is visible, an empty list is not).

The distinction matters: "no windows open" and "the enumeration API moved" are different states that today look identical to the user.

Discovered while verifying #127.

[jonocodes]

Implemented in 40a38e5 (Option 1: keep the [] fallback, log once per enable() when neither global.get_window_actors nor global.display.get_window_actors exists).

Actor-source selection is extracted into resolveWindowActors() in wire-shape.js (the compositor-free seam) and covered in scripts/test_focus_wire_shape.mjs across all three branches — preferred accessor, global.display fallback, and neither-present-warns-once. That missing-API branch was previously untestable, which is why the original regression stayed green.

Left open with human-verification-required: the warning path is proven by unit test but has not been observed in a real journal. To sign off: on a live GNOME session, confirm that when the enumeration API is genuinely absent, the running-windows list still shows its empty state and journalctl shows the [deckd-focus] ListWindows: no get_window_actors()... line exactly once per extension enable (not once per ~100ms poll). Remove the label and close once seen.