Route telemetry up and commands down over the cloud uplink

The proxy did not actually proxy. `fleet` was loaded and connected in
main.rs and then never touched again, while the uplink said hello and sent
heartbeats into the void - two halves of the program running side by side
with nothing between them. Both directions are wired now.

Up: every 5s, one vendor-neutral `PrinterTelemetry` snapshot per printer
that has reported, batched into a single `Telemetry` message. The snapshot
is a translation rather than a forwarded `print.push_status`, because it
has to describe a Prusa too, and because Bambu's wire format has traps
(0-15 fan gears, ints-or-floats temperatures) that should not reach the
rest of the system.

Down: a `Command` is looked up by our own printer id, executed, and
answered with a `CommandResult`. `PrinterCommand` is deliberately small and
deliberately has no "start a print" variant - `start_uploaded_file` has
never been run against hardware, and the first thing to exercise it should
be a human watching the machine, not a message from the internet.

The fleet becomes `Arc<Mutex<Vec<PrinterHandle>>>` so main and the uplink
can share it. One lock for the whole Vec is coarse - a slow command to one
printer stalls telemetry for all of them - which is fine at a handful of
printers and is the shape to revisit (a lock per printer, or an actor task
each) before a farm gets large.

Two things found by running this against the live P1P and H2C:

- A v1 Bambu reached the cloud as a row of nulls with one real bed
  temperature in it, while the v2 H2C beside it was fully populated. v1
  only sends a *complete* status when asked, and `pushall`'s own doc
  comment said to call it periodically - nothing ever did. Each telemetry
  tick now collects, then requests the next refresh, so v1 telemetry runs
  one tick behind instead of staying stuck on fragments. The quirk lives in
  `PrinterHandle::refresh_status` so the uplink does not have to know it.
- The third instance of the silent-swallow parse bug, at the old
  uplink/mod.rs:63: `if let Ok(server_msg) = serde_json::from_str(..)` with
  no else. Same shape that hid broken `print.push_status` parsing for
  weeks, except here it would mean commands vanishing with the cloud never
  being told. Now a `match` with a loud `Err` arm.

Verified end to end against both printers with a throwaway WebSocket
server: H2C `RUNNING 88% layer 48/50 13min nozzle 230/230 bed 35/35`, P1P
`IDLE 0% nozzle 23.5/0 bed 22.5/0`, both fan commands `ok=true`, zero
warnings over 24s.

src/uplink/protocol.rs

@@ -6,6 +6,56 @@ use serde::{Deserialize, Serialize};
 pub enum ClientMessage {
     Hello { gateway_id: String, farm_id: String, token: String },
     Heartbeat { seq: u64 },
+    /// One snapshot per printer, on a timer. Sent as a batch rather than a
+    /// message per printer: a farm has one uplink and many printers, and the
+    /// cloud wants a coherent picture of the farm at an instant more than it
+    /// wants the lowest possible latency per machine.
+    Telemetry { printers: Vec<PrinterTelemetry> },
+    /// The reply to a `ServerMessage::Command`. Correlated by `command_id`,
+    /// which the *cloud* generates - unlike Bambu's `sequence_id`, we're
+    /// both ends of this protocol, so the side that needs the correlation
+    /// gets to choose it.
+    CommandResult { command_id: String, ok: bool, error: Option<String> },
+}
+
+/// A flattened snapshot of one printer, in the terms an operator console
+/// actually displays - deliberately not Bambu's `print.push_status` shape.
+///
+/// Two reasons for the translation rather than forwarding the raw report:
+/// this has to describe a Prusa or a Klipper printer too, and Bambu's wire
+/// format has traps that should not reach the rest of the system (fan
+/// speeds are 0-15 gears, temperatures arrive as either integers or floats,
+/// and v2 reuses one `sequence_id` forever - see `PrintReport`).
+///
+/// Every field is optional because an unreachable or just-connected printer
+/// genuinely has nothing to report, and "no data yet" is different from
+/// "zero".
+#[derive(Debug, Clone, Serialize)]
+pub struct PrinterTelemetry {
+    /// Our own id from printers.toml, not the vendor serial - this is what
+    /// the cloud's `printers` table keys on.
+    pub printer_id: String,
+    pub name: String,
+    /// `"IDLE"`, `"RUNNING"`, `"PAUSE"`, `"FINISH"`, `"FAILED"` - passed
+    /// through as the printer's own string rather than narrowed to an enum,
+    /// so an unrecognised state reaches the cloud as itself instead of being
+    /// dropped on the floor here.
+    pub state: Option<String>,
+    pub percent: Option<i32>,
+    pub layer: Option<i32>,
+    pub total_layers: Option<i32>,
+    pub remaining_minutes: Option<i32>,
+    pub job_name: Option<String>,
+    pub nozzle_temp: Option<f32>,
+    pub nozzle_target: Option<f32>,
+    pub bed_temp: Option<f32>,
+    pub bed_target: Option<f32>,
+    /// Non-zero means the printer is reporting a fault.
+    pub print_error: Option<i32>,
+    /// How many Health-Management-System entries the printer currently has
+    /// active. Just the count for now - decoding the `{attr, code}` pairs
+    /// into messages needs Bambu's HMS tables, which we don't have yet.
+    pub active_alerts: Option<usize>,
 }
 
 /// What `continuum-backend` sends back over the same connection.
@@ -14,4 +64,25 @@ pub enum ClientMessage {
 pub enum ServerMessage {
     HelloAck { session_id: String },
     HeartbeatAck { seq: u64 },
+    /// Do something to one printer. `printer_id` is our own id from
+    /// printers.toml.
+    Command { command_id: String, printer_id: String, command: PrinterCommand },
+}
+
+/// The closed set of things the cloud is allowed to ask a printer to do.
+///
+/// Deliberately small, and deliberately missing the obvious one: there is no
+/// "start a print" variant. `BambuPrinter::start_uploaded_file` exists but
+/// has never been run against real hardware, and the first thing to exercise
+/// it should be a human watching the machine - not a message arriving from
+/// the internet. Add it here once it's been confirmed by hand.
+#[derive(Debug, Clone, Deserialize)]
+#[serde(tag = "action", rename_all = "snake_case")]
+pub enum PrinterCommand {
+    /// 0-100. Each vendor converts to whatever its own mechanism wants (for
+    /// Bambu, an `M106` PWM value over MQTT).
+    SetFanSpeed { percent: u8 },
+    /// A single raw gcode line. The most powerful variant by far, and the
+    /// one to be most careful about ever exposing to a UI.
+    SendGcodeLine { line: String },
 }