Load the client's Prepar3D TSV exports into the telemetry schema #228

Is your feature request related to a problem? Please describe.
features.py expects the logger schema (pilot_id, flight_id, altitude, heading, vertical_speed, velocity, target_*). The client's files were exported from Prepar3D, not X-Plane, and their headers change from file to file. Nothing in inference/ can read them.

What the client files look like (checked across all 19):

• 5 pilots (P001, P002, P003, P004, P007) and 19 flights, named P001_T1.tsv and so on
• Tab-separated, sampled at about 1 Hz, latin-1 encoded (the degree sign is mis-encoded)
• The header changes from file to file: Nose gear appears in 5 files (always 0), Flaps position percent is missing in P004_T1 and P004_T3, and 4 files carry a stray Engine N … column (Engine 2–4, every value 0)
• Latitude/Longitude appear twice, and every file ends in 2 empty columns

Describe the solution you'd like
Add a pure inference/prepar3d_export.py and a thin CLI load_client_data.py.

• Map columns by header name, never by position. Strip the unit suffix after the comma.
  • Altitude MSL → altitude (ft)
  • Heading (true) → heading (deg true; note that the schema doc says magnetic)
  • Indicated Airspeed → velocity (kts)
  • Vertical speed → vertical_speed (fpm)
  • Keep pitch, bank, throttle, elevator_input, aileron_input, rudder_input, elevator_trim and flaps (when present).
• Derive pilot_id and trial from the file name (P001_T3 → P001, 3) and set flight_id to P001_T3.
• Drop the duplicate Lat/Lon, the empty trailing columns and stray Engine N columns. Make flaps and nose_gear NaN when the column is missing, not 0.
• These files come from Prepar3D, not through our plugin, so the Plugin applies metric conversion factors to pilot altitude and airspeed that are already in feet and knots #196 rescaling does not apply. Sanity-check the units: cruise IAS should be about 70–130 kt and the field elevation near 1,100 ft.

Acceptance criteria

• All 19 files load into one long table with one row per sample
• Tests cover each header variant: Nose gear present, Flaps missing, stray Engine column
• A test checks that a missing required column raises an error naming it
• docs/telemetry_schema.md gains a section on the client export format and its mapping

Update (source of the data): the client says these files "were derived from Prepar3D, as I currently do not have X-Plane data", and that the plan is to build the prototype on them and migrate to X-Plane later. Keep this loader separate from the plugin/logger path: it is a loader for Prepar3D exports only. The plugin will be the source once the client moves to X-Plane.