Add start_print to the printer command set

Held back until there was a job path to drive it; the queueing work needs it
now. Three things are deliberate about its shape.

It carries a presigned `url`, not the file's bytes. A sliced project runs to
tens of megabytes and this is a JSON control channel, and a URL also keeps
object-store credentials out of every gateway on every farm. The schema says
to presign it at *dispatch* time rather than at job creation, because a queued
job may wait days and a URL minted when it was queued would be long expired.

It carries no filament or AMS field. The gateway reads the mapping out of the
archive's own `Metadata/slice_info.config`, because the archive is the only
thing that knows what each plate was sliced for. Passing one in would
reintroduce the guess that made a real P1P heat to 250C and pause itself at
1%.

And its description is blunt about what accepting it means, which is: very
little. A full end-to-end run on 2026-10-15 was accepted, uploaded
byte-perfect, reached RUNNING, and then the printer paused itself on a
physical broken-filament fault. Job state has to be driven by the telemetry
that follows, never by this reply - so the schema says so where a caller will
read it, rather than leaving it to be rediscovered.

packages/ts-types/src/schemas/edge.ts

@@ -75,6 +75,30 @@ export type PrinterTelemetry = Static<typeof PrinterTelemetrySchema>;
  * start-an-uploaded-file path has been run against real hardware by hand.
  */
 export const PrinterCommandSchema = Type.Union([
+  Type.Object({
+    action: Type.Literal("start_print"),
+    /**
+     * The cloud's `print_jobs.id`. The gateway does not need it to do the
+     * work - it is here so a gateway's logs line up with a job.
+     */
+    job_id: Type.String({ format: "uuid" }),
+    /**
+     * What to call the file on the printer's own storage. A `.3mf` is a
+     * sliced project and its filament mapping is read out of the archive;
+     * anything else is bare gcode and carries no mapping.
+     */
+    file_name: Type.String({ minLength: 1, maxLength: 500 }),
+    /**
+     * A presigned, expiring GET for the job file - a URL rather than the
+     * bytes, because the file is tens of megabytes and this is a JSON
+     * control channel, and because it keeps object-store credentials out of
+     * every gateway on every farm.
+     *
+     * Presign this at *dispatch* time, never at job creation: a queued job
+     * may wait days and the URL would be long expired.
+     */
+    url: Type.String({ format: "uri" }),
+  }),
   Type.Object({
     action: Type.Literal("set_fan_speed"),
     /**