Function load_nexus_bank_spectrum 

pub fn load_nexus_bank_spectrum(
    path: &Path,
    bank: &str,
    params: &BankBinningParams,
    keep_intervals: Option<&[(f64, f64)]>,
) -> Result<BankSpectrum, IoError>

Load one NXevent_data bank (e.g. a beam monitor) as a 1-D TOF spectrum, optionally keeping only pulses inside wall-clock keep_intervals (issue #637).

Reads /entry/<bank>/{event_time_offset, event_index, event_time_zero}:

• event_time_offset — TOF per event; its units attribute is required on this path (facility files always write it; refusing to guess closes the #554 silent-rescale class). Recognised values are the module-level table (ns/us/ms/s), scaled to canonical µs.
• event_index — cumulative first-event index per pulse (validated non-decreasing, last entry ≤ total events). Events of pulse p are event_index[p] .. event_index[p+1] (last pulse runs to the end).
• event_time_zero — pulse wall-clock times; its units attribute is also required (NXevent_data specifies no default; SNS writes "second"), accepted via the same table and rescaled to seconds.

keep_intervals are (t_start, t_end) pairs in seconds on the same clock as event_time_zero (at SNS: seconds since run start — the same clock as /entry/DASlogs/<pv>/time, so lists from crate::runlog::intervals_where / crate::runlog::intervals_intersect apply directly). Pulse p is kept iff t_start <= event_time_zero[p] < t_end for some interval; the list may be unsorted/overlapping (it is normalised internally), but every pair must be finite with t_end > t_start.

Empty-bank grace (issue #637): a bank with zero events (the normal state of every imaging-detector bank on VENUS, where tpx1 is frame-mode) loads to an all-zero spectrum with correct pulse statistics — it never errors.