hive-sh4re: one saturating_age for every loose-end producer

`age_seconds` is documented on the LooseEnd enum as saturating to zero on
any clock anomaly, but the derivation was in three places: hive-c0re had a
named `saturating_age` helper with tests, and the in-agent socket server
hand-rolled the same two lines twice, untested.

Move the helper to hive-sh4re::inbox, beside the enum whose contract it
implements and inside the one crate both producers already depend on. Its
three tests move with it (not dropped) and gain two arms: the whole-i64
range, where the saturating_sub is what stops the subtraction overflowing,
and a far-past control so those zeros are the clamp firing rather than the
function bottoming out on large inputs.

The two clamps are not redundant, which is what `to_loose_end`'s doc got
wrong: it credited "saturating" for the zero, but saturating_sub bottoms
out at i64::MIN, still negative. The try_from is what yields 0.

Also cover the two projections themselves, which is the part the shared
helper cannot: that a reminder ages from created_at rather than due_at,
and a todo from updated_at, with a future timestamp reading 0 through
both and a past-timestamp control on each.

hive-c0re/src/loose_ends.rs

@@ -20,7 +20,7 @@
 
 use anyhow::Result;
 use chrono::Utc;
-use hive_sh4re::inbox::LooseEnd;
+use hive_sh4re::inbox::{LooseEnd, saturating_age};
 
 use crate::coordinator::Coordinator;
 
@@ -93,32 +93,3 @@ pub fn hive_wide(coord: &Coordinator) -> Result<Vec<LooseEnd>> {
     }
     Ok(out)
 }
-
-fn saturating_age(now: i64, then: i64) -> u64 {
-    let delta = now.saturating_sub(then);
-    u64::try_from(delta).unwrap_or(0)
-}
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    #[test]
-    fn saturating_age_handles_clock_back_step() {
-        // `now` < `then`: caller's clock went backwards between rows.
-        // We saturate to 0 rather than returning a negative or
-        // wrapping around to ~u64::MAX (which would render as "27
-        // billion years ago" in the wake prompt).
-        assert_eq!(saturating_age(100, 200), 0);
-    }
-
-    #[test]
-    fn saturating_age_normal_case() {
-        assert_eq!(saturating_age(1_000_000, 999_990), 10);
-    }
-
-    #[test]
-    fn saturating_age_zero_when_equal() {
-        assert_eq!(saturating_age(42, 42), 0);
-    }
-}