Add DECODER_REGISTRY in trx-protocol::decoders as the single source of
truth for all decoder metadata (activation mode, supported rig modes,
background-decode capability). Replace duplicated resolver functions in
background_decode.rs and sse.rs with shared resolve_bookmark_decoders().
Add GET /decoders endpoint to expose the registry to the frontend.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>
