replace libc date math with chrono, extract memory_subdir helper

- date_to_epoch, iso_week_info, weeks_in_month: replaced unsafe libc
  (mktime, strftime, localtime_r) with chrono NaiveDate and IsoWeek
- epoch_to_local: replaced unsafe libc localtime_r with chrono Local
- New util.rs with memory_subdir() helper: ensures subdir exists and
  propagates errors instead of silently ignoring them
- Removed three duplicate agent_results_dir() definitions across
  digest.rs, consolidate.rs, enrich.rs
- load_digest_files, parse_all_digest_links, find_consolidation_reports
  now return Result to properly propagate directory creation errors

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

src/store/types.rs

@@ -130,17 +130,15 @@ pub fn now_epoch() -> f64 {
 /// Convert epoch seconds to broken-down local time components.
 /// Returns (year, month, day, hour, minute, second).
 pub fn epoch_to_local(epoch: f64) -> (i32, u32, u32, u32, u32, u32) {
-    // Use libc localtime_r for timezone-correct conversion
-    let secs = epoch as libc::time_t;
-    let mut tm: libc::tm = unsafe { std::mem::zeroed() };
-    unsafe { libc::localtime_r(&secs, &mut tm) };
+    use chrono::{Datelike, Local, TimeZone, Timelike};
+    let dt = Local.timestamp_opt(epoch as i64, 0).unwrap();
     (
-        tm.tm_year + 1900,
-        (tm.tm_mon + 1) as u32,
-        tm.tm_mday as u32,
-        tm.tm_hour as u32,
-        tm.tm_min as u32,
-        tm.tm_sec as u32,
+        dt.year(),
+        dt.month(),
+        dt.day(),
+        dt.hour(),
+        dt.minute(),
+        dt.second(),
     )
 }