Stop the keybindings menu's config scan from hanging

What

The keybindings menu reads the user's Hyprland config for the binds Hyprland reports as Lua. It does that with a stub of Hyprland's Lua API, so the config can be loaded without a compositor running.

The stub answered every index, so a config that walks a result set while it loads (ipairs(hl.get_monitors())) never finished: ipairs over a table that answers every index is an endless loop. The menu then waited on the scan, and the scan sat at 100% CPU. The chord for the keybindings menu did nothing.

The fix

• The stub answers a numeric index with nil, which is the empty result set such a config has to see.
• The scan runs under a budget (10 seconds; OMARCHY_KEYBINDINGS_SCAN_TIMEOUT overrides it). The scan only fills in what Hyprland leaves out, so a config that never finishes loading must not keep the menu from opening: the menu falls back to the binds Hyprland reports, which are the truth anyway.

Tests

test/shell.d/keybindings-menu-config-scan-test.sh covers both halves: a config that walks a result set still reaches the binds declared after it, and a config that never finishes loading still opens the menu.