Declare 80 slots in one line

Multi-slot procedures no longer need one YAML entry per slot.

• Added an integer shorthand for execution.slots: slots: 80 expands to slots slot_01 to slot_80, zero-padded so they sort in order everywhere
• Kept the list form for custom slot keys and names
• Improved load errors: a wrong count or type says what was expected, and a bad entry in the list keeps its own message
• Updated Studio to open procedures that use the shorthand