Adds optional `@N` path suffix to FocasAddress (PARAM:1815@2, R100@3.0,
MACRO:500@2, DIAG:280@2/1) with PathId defaulting to 1 for back-compat.
Per-device PathCount is discovered via cnc_rdpathnum at first connect and
cached on DeviceState; reads with PathId>PathCount return BadOutOfRange.
The driver issues cnc_setpath before each non-default-path read and
tracks LastSetPath so repeat reads on the same path skip the wire call.
Closes#264
