Closes#259
Adds Modal/ + Override/ fixed-tree subfolders per FOCAS device, mirroring the
pattern established by Status/ (#257) and Production/ (#258): cached snapshots
refreshed on the probe tick, served from cache on read, no extra wire traffic
on top of user-driven tag reads.
Modal/ surfaces the four universally-present aux modal codes M/S/T/B from
cnc_modal(type=100..103) as Int16. **G-group decoding (groups 1..21) is deferred
to a follow-up** — the FWLIB ODBMDL union differs per series + group and the
issue body explicitly permits this scoping. Adds the cnc_modal P/Invoke +
ODBMDL struct + a generic int16 cnc_rdparam helper so the follow-up can add
G-groups without further wire-level scaffolding.
Override/ surfaces Feed/Rapid/Spindle/Jog from cnc_rdparam at MTB-specific
parameter numbers (FocasDeviceOptions.OverrideParameters; defaults to 30i:
6010/6011/6014/6015). Per-field nullable params let a deployment hide overrides
their MTB doesn't wire up; passing OverrideParameters=null suppresses the entire
Override/ subfolder for that device.
6 unit tests cover discovery shape, omitted Override folder when unconfigured,
partial Override field selection, cached-snapshot reads (Modal + Override),
BadCommunicationError before first refresh, and the FwlibFocasClient
disconnected short-circuit.
