Make the filament source explicit, and print from the AMS

`start_uploaded_file` hardcoded `use_ams: false` / `ams_mapping: ""`, which
silently overrode whatever the slicer had chosen. On a P1P with an AMS and a
project sliced for it, that is not a missing feature - it points the print at
the external spool. Confirmed on 2026-10-11: the print started, heated to
250C, reached 1%, then the printer paused itself with `print_error: 50364420`
and `tray_now: 255`. Nothing in the command's reply said so; `result` was
`success`, because the command had been accepted.

So `FilamentSource` is now a required argument with no default, because there
is no safe default - picking wrong draws the wrong filament or none. The
`ExternalSpool` variant sends the empty *string* the schema's trailing
`oneOf` demands, and `ams` takes slots per project filament index and pads to
every tray the printer has, per the padding rule in
`schemas/bambulab/v1/common/ams/ams_mapping.schema.json`. It refuses an empty
mapping (padding `[]` would mean `use_ams: true` with nothing mapped), one
longer than the printer has trays, and any negative that is not -1.

Confirmed live with the real mapping: `ams_mapping: [3,-1,-1,-1,-1]`,
`use_ams: true` took the printer to RUNNING, `tray_now` became 3 - the slot
the project was sliced for - and it ran past 1% to 13% with `print_error: 0`,
where the external-spool attempt had paused.

Finding the mapping needed reading the project back off the card, so FTPS
gains `download`/`download_file` and the probe a `fetch` step. The archive's
`Metadata/slice_info.config` lists each plate's filaments with
`tray_info_idx` and colour - here one PETG GFG99 at `#2850E0`, matching AMS
slot 3's `2850E0FF` - and `Metadata/plate_1.gcode` is present, which also
confirms the hardcoded `param`.

`upload_project_file` is left on the external spool and is the one remaining
place that can pick wrong, because it is vendor-generic and an AMS mapping is
a Bambu concept. It is unreachable - there is no start-print command on the
uplink - and the fix is for it to derive the mapping from the `.3mf` it
already holds, which needs a zip reader and so a dependency decision.

src/printer/mod.rs

@@ -35,7 +35,9 @@ mod klipper;
 mod marlin_serial;
 mod prusa;
 
-pub use bambu::{BambuPrinter, BambuProtocolPreference, BambuProtocolVersion, BambuTls};
+pub use bambu::{
+    BambuPrinter, BambuProtocolPreference, BambuProtocolVersion, BambuTls, FilamentSource,
+};
 pub use bambu_commands::{
     AmsState, AmsTray, AmsUnit, BambuReport, BambuRequest, CommandEnvelope, EmptyTray, GetVersionRequest,
     GetVersionResponse, LoadedTray, ModuleInfo, PrintReport, PushallRequest, XcamSettings,