Hide non-writable channels from Controls picker, gate writes on enabled+writable

Adds ChannelConfig.writable so the Controls editor can only bind to
channels with a real write mapping (VS/MA/ME/MD/ST for CML, do*/DO for
digital I/O, write_cmd-configured SCPI channels, holding-register Modbus
channels) instead of read-only telemetry — this is exactly the class of
mistake hit earlier (binding a motor-speed control to M1_TV instead of
M1_VS). Also carries forward the enabled-channel gating for controls
(hide disabled channels from the picker, block writes to them at
_write() time) done alongside this investigation.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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