Bug Fixes: Settings loading and livetime validation

- `spectroscopy` calls `load_panda_settings` with a selective whitelist, then `fly_scan` calls it again with no whitelist.
- No guard against negative scan_frame_livetime. scan_frame_livetime = exposure_time - detector_deadtime. If exposure_time <= 2e-3, livetime is negative, there should be a `ValueError`

## Acceptance Criteria
- PandA settings are loaded exactly once, in one place, with the correct whitelist
- A `ValueError` raised if `exposure_time <= detector_deadtime` at scan setup

Trace the spectroscopy and fly_scan entry points, focusing on their calls to load_panda_settings and the scan setup calculation for scan_frame_livetime. Confirm where PandA settings are loaded and where exposure_time and detector_deadtime are available. Done means one whitelisted settings load and a ValueError when exposure_time is less than or equal to detector_deadtime.

Written by the indexing model from the issue text.