load yaml-defined detectors from ~/.config/sif/signatures (AppData\Local
on windows), mirroring the user-modules convention, so a framework sif
does not ship can be detected without a rebuild. they load lazily once
per run from DetectFramework and register alongside the built-ins.
each file is one detector, scored by the same weighted signature match as
the built-ins. confidence is linear rather than their sigmoid (importing
it would cycle), so a detector clears the 0.5 threshold once its matched
weights pass half. a name matching a built-in overrides it and inherits
that built-in's version patterns and cves, the same as a user module. a
single unparseable file warns and is skipped rather than failing the scan.
implements the custom signature support help-wanted item in contributing.
