[feat](trx-rs): make spectrum affordable over a slow link

Spectrum dominates the server↔client connection, and all three things that
govern its cost were working against a poor link.

**It was polled, one round trip per frame.** The client asked for a frame every
50 ms on a dedicated connection and waited for the reply, so the frame rate was
capped at 1/RTT — on a 200 ms link, five frames a second no matter what was
configured.  Add SubscribeSpectrum alongside the existing SubscribeMeter: the
server pushes frames from a per-rig broadcast that rig_task fills only while
somebody is subscribed.  A server too old to know the command answers with an
error and leaves the connection usable, so the client falls back to polling on
the same connection without reconnecting.

**Bins were JSON floats.** 1024 bins spelled out as decimal text is around
10 KB a frame, ~200 KB/s at full rate — while the very next hop, client to
browser, already sends the same information as base64 i8 in about 1.4 KB.  Bins
now travel base64-encoded whole dBFS, the resolution the display draws at
anyway.  Decoding still accepts the old array form.

**Nothing was tunable.** [sdr].spectrum_fft_size and [sdr].spectrum_interval_ms
replace the compile-time FFT size and cadence; [[remotes]].spectrum_interval_ms
lets the client ask for less.  512 bins at 5 frames/s is roughly 3.5 KB/s
against roughly 200 KB/s before.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SyX26FCpMQxiBoC7r5K1A7
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-protocol/src/lib.rs

@@ -18,4 +18,6 @@ pub use auth::{NoAuthValidator, SimpleTokenValidator, TokenValidator};
 pub use codec::{mode_to_string, parse_envelope, parse_mode};
 pub use decoders::{DecoderActivation, DecoderDescriptor, DECODER_REGISTRY};
 pub use mapping::{client_command_to_rig, rig_command_to_client};
-pub use types::{ClientCommand, ClientEnvelope, ClientResponse, MeterUpdate, RigEntry};
+pub use types::{
+    ClientCommand, ClientEnvelope, ClientResponse, MeterUpdate, RigEntry, SpectrumFrame,
+};

src/trx-protocol/src/mapping.rs

@@ -102,7 +102,7 @@ macro_rules! define_command_mapping {
 
 define_command_mapping! {
     // ── Client-only variants (no RigCommand counterpart) ─────────────
-    client_only: GetRigs, GetSatPasses, SubscribeMeter;
+    client_only: GetRigs, GetSatPasses, SubscribeMeter, SubscribeSpectrum;
 
     // ── Unit variants (no payload) ───────────────────────────────────
     unit:

src/trx-protocol/src/types.rs

@@ -130,6 +130,28 @@ pub enum ClientCommand {
     /// newline-delimited `MeterUpdate` JSON frames and no further commands or
     /// regular responses are sent.  Intended for a dedicated TCP connection.
     SubscribeMeter,
+    /// Subscribe to a per-rig spectrum stream on this connection.  Like
+    /// `SubscribeMeter`, the connection becomes a one-way flow of
+    /// newline-delimited `SpectrumFrame` JSON and no further commands or
+    /// regular responses are sent.
+    ///
+    /// Polling `GetSpectrum` costs a round trip per frame, which caps the frame
+    /// rate at 1/RTT no matter how often the client asks; the server pushes at
+    /// its own cadence instead.  Clients fall back to polling when the server
+    /// rejects this command.
+    SubscribeSpectrum,
+}
+
+/// One spectrum frame pushed by the server on a dedicated spectrum stream.
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
+pub struct SpectrumFrame {
+    /// Rig identifier this frame belongs to.
+    pub rig_id: String,
+    /// The frame itself; bins travel base64-encoded (see `spectrum_wire`).
+    pub spectrum: trx_core::rig::state::SpectrumData,
+    /// Virtual-channel RDS state, mirroring what `GetSpectrum` returned.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub vchan_rds: Option<Vec<trx_core::rig::state::VchanRdsEntry>>,
 }
 
 /// Fast meter sample pushed by the server on a dedicated meter stream.