[feat](trx-rs): add FT2 decoder support (wired to FT4)
Mirrors the FT4 implementation across the full stack. The trx-ft8
crate wires Ft8Decoder::new_ft2() to FTX_PROTOCOL_FT4 as a
placeholder pending a dedicated FT2 implementation.
Changes:
- trx-ft8: Ft8Decoder::new_ft2() delegates to with_protocol(Ft4)
- trx-core: DecodedMessage::Ft2, AUDIO_MSG_FT2_DECODE (0x15),
ft2_decode_enabled/ft2_decode_reset_seq state, SetFt2DecodeEnabled/
ResetFt2Decoder commands, protocol mapping
- trx-server: DecoderHistories::ft2, run_ft2_decoder (7.5s slots),
run_background_ft2_decoder, history push/replay, decoder task spawn
- trx-frontend-http: ft2_history in FrontendRuntimeContext,
toggle/clear endpoints, /ft2.js route, bookmark/scheduler/background
decode support, DecodeHistoryPayload ft2 field
- web: ft2.js plugin (3.75s period timer), FT2 subtab in index.html,
FT2 map source (distinct hue), app.js dispatch, decode-history-worker
HISTORY_GROUP_KEYS, bookmarks read/write/apply
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>
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