Reject a duration with characters outside its parts (closes #215)

parseDuration fell back to an unanchored search for number-and-unit
pieces when time.ParseDuration failed, and skipped everything in
between. 1.0y became 0, so `snapshot create --prune --keep-newer-than
1.0y` deleted every snapshot of the backed-up names, the new one
included. 2.1w became one week and 1,5y five years. The fallback now
requires the whole input to be whole-number-and-unit parts with nothing
between them. A bare number is rejected before time.ParseDuration sees
it, since Go reads 0 and +0 as zero with no unit.

Judgement call: a space between number and unit (`30 days`) was
accepted and is now an error, matching Go's own units.

Model: opus-5-5

TODO.md

@@ -22,6 +22,15 @@ the tag exists and is exercised; what is left is merging `next` to
 
 # Completed Steps
 
+- 2026-10-06: Made `--older-than` and `--keep-newer-than` reject a
+  duration with characters outside its number-and-unit parts
+  ([issue #215](https://git.eeqj.de/sneak/vaultik/issues/215)). The
+  parser picked out the parts it recognised and skipped the rest, so
+  `1.0y` became zero and `--prune --keep-newer-than 1.0y` deleted every
+  snapshot of the backed-up names, the new one included. `1.0y`,
+  `1,5y`, `30 days`, `x7d` and a bare `0` are now errors; decimals still
+  work in Go units such as `1.5h`.
+
 - 2026-10-06: Made a backup re-chunk a known file that lists a chunk no
   uploaded blob holds
   ([issue #214](https://git.eeqj.de/sneak/vaultik/issues/214)). File