fix(hot-corner): use usable monitor corners

[inayayousfi]

Written by inayayousfi, typed by gpt-5.6-sol running in OpenCode.
Every call here is inayayousfi's, and no agent acted on its own.

Summary

• Anchor Wayland hot-corner helpers to their matching screen edges so compositors can move them around panels with exclusive zones.
• Use available monitor geometry for X11 and native cursor polling, and ignore reserved pixels outside that area.
• Keep dwell indicators aligned with the usable corners and add regression coverage for both placement paths.

Architecture

This keeps hot-corner placement inside the existing isolated overlay controller. Runtime events, corner IDs, bindings, and window orchestration do not change.

Testing

• .venv-windows/Scripts/python.exe -m flake8 --select=F,E9,W6 src
• .venv-windows/Scripts/python.exe -m unittest discover -s tests (169 passed, 1 skipped)