refactor(core/time): extract parse_since_from for deterministic time parsing

Factor out parse_since_from(input, reference_ms) so callers can compute
relative durations against a fixed reference timestamp instead of always
using now(). The existing parse_since() now delegates to it with now_ms().

Enables testable and reproducible time-relative queries for features like
timeline --as-of and who --as-of.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/core/time.rs

@@ -17,21 +17,27 @@ pub fn now_ms() -> i64 {
 }
 
 pub fn parse_since(input: &str) -> Option<i64> {
+    parse_since_from(input, now_ms())
+}
+
+/// Like `parse_since` but durations are relative to `reference_ms` instead of now.
+/// Absolute dates/timestamps are returned as-is regardless of `reference_ms`.
+pub fn parse_since_from(input: &str, reference_ms: i64) -> Option<i64> {
     let input = input.trim();
 
     if let Some(num_str) = input.strip_suffix('d') {
         let days: i64 = num_str.parse().ok()?;
-        return Some(now_ms() - (days * 24 * 60 * 60 * 1000));
+        return Some(reference_ms - (days * 24 * 60 * 60 * 1000));
     }
 
     if let Some(num_str) = input.strip_suffix('w') {
         let weeks: i64 = num_str.parse().ok()?;
-        return Some(now_ms() - (weeks * 7 * 24 * 60 * 60 * 1000));
+        return Some(reference_ms - (weeks * 7 * 24 * 60 * 60 * 1000));
     }
 
     if let Some(num_str) = input.strip_suffix('m') {
         let months: i64 = num_str.parse().ok()?;
-        return Some(now_ms() - (months * 30 * 24 * 60 * 60 * 1000));
+        return Some(reference_ms - (months * 30 * 24 * 60 * 60 * 1000));
     }
 
     if input.len() == 10 && input.chars().filter(|&c| c == '-').count() == 2 {