Bundled pure-Python fallback modules for CFWs whose system Python lacks
HAC's dependencies (muOS 2601.0 ships no dbus/evdev/gi — reports
20260709201641-y2zx, 20260709202224-nehf).

_hac_fallback/ contains from-scratch implementations of the subsets HAC
uses:
  dbus/   — D-Bus wire-protocol client (SASL EXTERNAL, marshalling,
            blocking calls, exported Profile1/Agent1 objects, unix-fd
            receive). Method signatures for the BlueZ/freedesktop calls
            are hardcoded in _METHOD_SIGS (no introspection).
  evdev/  — InputDevice via raw ioctls + input_event struct parsing.
  gi/     — GLib.MainLoop run/quit stub.

hac_main.py first imports the real modules; only when that fails does it
append this directory to sys.path and retry, so platforms with the real
C extensions are never affected. The startup log line
"Fallback shims active for: [...]" shows when these are in use.

Tests: tools/test_hac_fallback.py (fake bus daemon over a socketpair).
