digest: unify gather/find with composable date_range + date_to_label

Each DigestLevel now carries two date-math fn pointers:
- label_dates: expand an arg into (label, dates covered)
- date_to_label: map any date to this level's label

Parent gather works by expanding its date range then mapping those
dates through the child level's date_to_label to derive child labels.
find_candidates groups journal dates through date_to_label and skips
the current period. This eliminates six per-level functions
(gather_daily/weekly/monthly, find_daily/weekly/monthly_args) and the
three generate_daily/weekly/monthly public entry points in favor of
one generic gather, one generic find_candidates, and one public
generate(store, level_name, arg).

src/main.rs

@@ -765,27 +765,15 @@ fn cmd_digest(args: &[String]) -> Result<(), String> {
     let date_arg = args.get(1).map(|s| s.as_str()).unwrap_or("");
 
     match args[0].as_str() {
-        "daily" => {
-            let date = if date_arg.is_empty() {
-                store::format_date(store::now_epoch())
-            } else {
-                date_arg.to_string()
-            };
-            digest::generate_daily(&mut store, &date)
-        }
-        "weekly" => {
-            let date = if date_arg.is_empty() {
-                store::format_date(store::now_epoch())
-            } else {
-                date_arg.to_string()
-            };
-            digest::generate_weekly(&mut store, &date)
-        }
-        "monthly" => {
-            let month = if date_arg.is_empty() { "" } else { date_arg };
-            digest::generate_monthly(&mut store, month)
-        }
         "auto" => digest::digest_auto(&mut store),
+        name @ ("daily" | "weekly" | "monthly") => {
+            let arg = if date_arg.is_empty() {
+                store::format_date(store::now_epoch())
+            } else {
+                date_arg.to_string()
+            };
+            digest::generate(&mut store, name, &arg)
+        }
         _ => Err(format!("Unknown digest type: {}. Use: daily, weekly, monthly, auto", args[0])),
     }
 }