Every parser already returned nil on malformed input, so the ||
chains fell through to defaults — except parse_time_param, which
happily accepted any well-formed ISO8601 even if it pointed
outside the current HRRR forecast window (e.g. a stale shared
link from last week). Gate it behind the valid-times window and
fall back to the normal 'now' cursor when it's out of range.
Adds explicit coverage: non-numeric band, unknown band MHz,
non-numeric lat/lon/zoom, out-of-range values, malformed time,
ancient time, and all-params-garbage-at-once. Each case mounts
cleanly and lands on the defaults.