Add HLD describing precoding configuration for SPC-6 platforms while
APSU is not yet supported by module firmware. The flow reuses the
standard SONiC SI pipeline:

  media_settings.json -> xcvrd -> APP_DB:PORT_TABLE -> orchagent -> SAI

- Two new per-lane fields, tx_precoding / rx_precoding, are added to
  media_settings.json under the existing speed:<lane_speed_key> blocks.
- xcvrd requires no code change (parser is field-agnostic).
- orchagent (PortsOrch) maps the fields to
  SAI_PORT_SERDES_ATTR_{TX,RX}_PRECODING.
- sonic-device-data's media_checker is extended to accept the new keys
  so the build-time validator does not reject them.

Tracking: FR #4867218.