Add missing docstring, remove dead code

The following literals should become named module-level constants with source comments:

```
2e-3 * 1.01 → ARAVIS_DEADTIME_S, DEADTIME_SAFETY_MARGIN
1961e-6 → ARAVIS_ACQUIRE_PERIOD_OVERHEAD_S
10 * 0.98 → MAX_STAGE_VELOCITY_MM_S, VELOCITY_SAFETY_MARGIN
```

Remove commented-out dead code in inner_plan
e.g '# Hashable prepare_group = ["pmac_trigger_logic", "trigger_logic"]` Remove it.

Missing docstring on fly_scan
All other plans have docstrings; `fly_scan` does not. BlueAPI surfaces docstrings as user-facing plan descriptions. Add a docstring explaining the purpose, parameters, and the required kickoff ordering.

Search the Python plans for inner_plan and fly_scan, then inspect the surrounding literal calculations and existing plan docstrings. Done means the listed literals use named module-level constants with source comments, the commented-out Hashable line is gone, and fly_scan documents its purpose, parameters, and required kickoff ordering.

Written by the indexing model from the issue text.