Fix polars 1.36.0 duration API

Polars tightened type safety for `.dt` accessor methods requiring
`total_*` methods for duration types vs datetime component accessors
like `day()` which now only work on datetime dtypes.

`detect_time_gaps()` in `.tsp._anal` was calling `.dt.day()`
on `dt_diff` column (a duration from `.diff()`) which throws
`InvalidOperationError` on modern polars.

Changes:
- use f-string to add pluralization to map time unit strings to
  `total_<unit>s` form for the new duration API.
- Handle singular/plural forms: 'day' -> 'days' -> 'total_days'
- Ensure trailing 's' before applying 'total_' prefix

Also updates inline comments explaining the polars type distinction
between datetime components vs duration totals.

Fixes `piker store ldshm` crashes on datasets with time gaps.

(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

piker/tsp/_anal.py

@@ -578,11 +578,22 @@ def detect_time_gaps(
     # NOTE: this flag is to indicate that on this (sampling) time
     # scale we expect to only be filtering against larger venue
     # closures-scale time gaps.
+    #
+    # Map to total_ method since `dt_diff` is a duration type,
+    # not datetime - modern polars requires `total_*` methods
+    # for duration types (e.g. `total_days()` not `day()`)
+    # Ensure plural form for polars API (e.g. 'day' -> 'days')
+    unit_plural: str = (
+        gap_dt_unit
+        if gap_dt_unit.endswith('s')
+        else f'{gap_dt_unit}s'
+    )
+    duration_method: str = f'total_{unit_plural}'
     return step_gaps.filter(
         # Second by an arbitrary dt-unit step size
         getattr(
             pl.col('dt_diff').dt,
-            gap_dt_unit,
+            duration_method,
         )().abs() > gap_thresh
     )