Channel Variables: restore media_mix_inbound_outbound_codecs and add 234 read-variables from source audit

Restores media_mix_inbound_outbound_codecs (dropped in the legacy
migration, PR #305) and adds 234 additional channel variables that
FreeSWITCH reads via switch_channel_get_variable*, found by auditing the
C source and diffing against the catalog. Each entry is grounded in a
source file. Read-back/informational, recovery-internal, debug-only, and
misspelled-alias variables were excluded; module-specific variables
remain deferred to their module-reference pages.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T3xbkfCyJTw9msgsNXyRBk

Author: briankwest