Upload one run per slot on multi-slot stations

A station testing several units at once used to upload a single run for the whole fixture, so per-unit results and per-slot yield were invisible.

• Added: one run per slot on every start, each with its own unit, outcome, phases, measurements, logs and attachments; the runs of one start share an execution id and carry their slot key and name (needs a dashboard at 2.42.2 or later; single-slot uploads are unchanged)
• Added: {slot} and {slot_name} placeholders in unit default values, so auto_identify derives each slot's serial number and a burn-in rack starts with no operator input
• Added: execution_id on run_started and per-slot outcomes on run_finished in the agent protocol, per-slot phase rows and prompts in the terminal UI
• Updated: Rust client with the run grouping fields and filters; docs on slot placeholders and one run per slot