feat(dates): pre-cutover audit utility for the filename date class (#117)

One-shot, operator-run, strictly read-only utility that lists every
filename-class file with its stored date/source and the date/source/
offset-source the re-based resolver would assign it — the human
spot-check that gates the production backfill drain (#118).

- src/filename_audit.rs: StoredDate/AuditRow/SkippedLibrary; walk +
  PathExcluder (depth-0 guarded) + match_filename_date -> one
  resolve_dates_batch pass with EMPTY priors (mirrors the drain
  exactly); disabled and probe-stale libraries reported as skipped,
  never partially processed; output stable-sorted by (library, path).
- src/bin/filename_audit.rs: operator binary (dotenv, DATABASE_URL,
  EXCLUDED_DIRS, libraries::load_all, format_report).
- src/memories.rs: match_filename_date canonical matcher returning
  FilenameMatch { pattern, date }; extract_date_from_filename is now
  a thin wrapper (byte-for-byte behavior-preserving, verified by
  review).

4 audit tests (inversion + utc_direct + exclusions, offline/disabled
skip, stable sort, report determinism incl. no debug quotes) + 2
matcher tests. 645 lib + 718 integration pass; clippy/fmt clean.
Standards review applied: {:?}->{}, depth-0 walk guard, dead
per-file exclude check removed, unused Default derive removed,
pattern column widened, doc comment reworded.

src/memories.rs

@@ -146,7 +146,7 @@ pub struct MemoriesResponse {
 /// class**: the filename is the original date of record and outranks
 /// EXIF / container dates (ADR-0001). Files whose name matches nothing
 /// are out of the class and keep the global EXIF-wins waterfall.
-#[derive(Copy, Clone, Debug, PartialEq)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
 pub enum FilenameDate {
     /// Wall clock the device displayed (screenshot / dash / compact).
     Wall(NaiveDateTime),
@@ -174,12 +174,26 @@ impl FilenameDate {
     }
 }
 
-pub fn extract_date_from_filename(filename: &str) -> Option<FilenameDate> {
+/// A filename that matched one of the filename-class patterns (#116):
+/// which pattern fired and the date it carries.
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub struct FilenameMatch {
+    /// Stable pattern name for audit / debug surfaces: `screenshot`,
+    /// `screenshot_compact`, `dash`, `dash_compact`, `compact`,
+    /// `unix_timestamp`.
+    pub pattern: &'static str,
+    pub date: FilenameDate,
+}
+
+/// Match a filename against the filename-class patterns, reporting which
+/// pattern fired. This is the canonical matcher; `extract_date_from_filename`
+/// is the date-only projection of it.
+pub fn match_filename_date(filename: &str) -> Option<FilenameMatch> {
     // The wall-clock patterns (screenshot / dash / compact) carry the
     // device's own clock; the unix-timestamp pattern carries a true
-    // instant. `legacy_datetime()` reproduces the pre-migration
-    // interpretation of each (naive-as-UTC / epoch) for the legacy
-    // column and its consumers.
+    // instant. `FilenameDate::legacy_datetime()` reproduces the
+    // pre-migration interpretation of each (naive-as-UTC / epoch) for the
+    // legacy column and its consumers.
     let build_wall_from_ymd_capture = |captures: &regex::Captures| -> Option<NaiveDateTime> {
         let year = captures.get(1)?.as_str().parse::<i32>().ok()?;
         let month = captures.get(2)?.as_str().parse::<u32>().ok()?;
@@ -197,7 +211,10 @@ pub fn extract_date_from_filename(filename: &str) -> Option<FilenameDate> {
         .captures(filename)
         .and_then(|c| build_wall_from_ymd_capture(&c))
     {
-        return Some(FilenameDate::Wall(naive));
+        return Some(FilenameMatch {
+            pattern: "screenshot",
+            date: FilenameDate::Wall(naive),
+        });
     }
 
     // Screenshot format: Screenshot_20140601[_-]204450.png
@@ -206,7 +223,10 @@ pub fn extract_date_from_filename(filename: &str) -> Option<FilenameDate> {
         .captures(filename)
         .and_then(|c| build_wall_from_ymd_capture(&c))
     {
-        return Some(FilenameDate::Wall(naive));
+        return Some(FilenameMatch {
+            pattern: "screenshot_compact",
+            date: FilenameDate::Wall(naive),
+        });
     }
 
     // 2. Dash format: 2015-01-09_02-15-15.jpg
@@ -215,7 +235,10 @@ pub fn extract_date_from_filename(filename: &str) -> Option<FilenameDate> {
         .captures(filename)
         .and_then(|c| build_wall_from_ymd_capture(&c))
     {
-        return Some(FilenameDate::Wall(naive));
+        return Some(FilenameMatch {
+            pattern: "dash",
+            date: FilenameDate::Wall(naive),
+        });
     }
 
     // Dash with compact time format: 2015-01-09-021515.jpg
@@ -224,7 +247,10 @@ pub fn extract_date_from_filename(filename: &str) -> Option<FilenameDate> {
         .captures(filename)
         .and_then(|c| build_wall_from_ymd_capture(&c))
     {
-        return Some(FilenameDate::Wall(naive));
+        return Some(FilenameMatch {
+            pattern: "dash_compact",
+            date: FilenameDate::Wall(naive),
+        });
     }
 
     // 3. Compact format: 20140927101712.jpg
@@ -233,7 +259,10 @@ pub fn extract_date_from_filename(filename: &str) -> Option<FilenameDate> {
         .captures(filename)
         .and_then(|c| build_wall_from_ymd_capture(&c))
     {
-        return Some(FilenameDate::Wall(naive));
+        return Some(FilenameMatch {
+            pattern: "compact",
+            date: FilenameDate::Wall(naive),
+        });
     }
 
     // 4. Timestamp format: 1401638400.jpeg, att_1422489664680106.jpeg, att_142248967186928.jpeg
@@ -281,7 +310,10 @@ pub fn extract_date_from_filename(filename: &str) -> Option<FilenameDate> {
                 .and_then(DateTime::<Utc>::from_timestamp_millis)
                 .and_then(plausible_filename_date)
         {
-            return Some(FilenameDate::Utc(date_time));
+            return Some(FilenameMatch {
+                pattern: "unix_timestamp",
+                date: FilenameDate::Utc(date_time),
+            });
         }
 
         // For 14-16 digits, treat first 10 digits as seconds to avoid far future dates
@@ -293,7 +325,10 @@ pub fn extract_date_from_filename(filename: &str) -> Option<FilenameDate> {
                 .and_then(|ts| DateTime::<Utc>::from_timestamp(ts, 0))
                 .and_then(plausible_filename_date)
         {
-            return Some(FilenameDate::Utc(date_time));
+            return Some(FilenameMatch {
+                pattern: "unix_timestamp",
+                date: FilenameDate::Utc(date_time),
+            });
         }
 
         // Exactly 10 digits - seconds since epoch
@@ -311,7 +346,10 @@ pub fn extract_date_from_filename(filename: &str) -> Option<FilenameDate> {
             if is_snapchat && date_time.timestamp() < SNAPCHAT_LAUNCH_TS {
                 return None;
             }
-            return Some(FilenameDate::Utc(date_time));
+            return Some(FilenameMatch {
+                pattern: "unix_timestamp",
+                date: FilenameDate::Utc(date_time),
+            });
         }
 
         // 11-12 digits: try as milliseconds (might be partial millisecond timestamp)
@@ -322,13 +360,23 @@ pub fn extract_date_from_filename(filename: &str) -> Option<FilenameDate> {
                 .and_then(DateTime::<Utc>::from_timestamp_millis)
                 .and_then(plausible_filename_date)
         {
-            return Some(FilenameDate::Utc(date_time));
+            return Some(FilenameMatch {
+                pattern: "unix_timestamp",
+                date: FilenameDate::Utc(date_time),
+            });
         }
     }
 
     None
 }
 
+/// Date-only projection of [`match_filename_date`]: the class match
+/// without the pattern name. Resolver and consumer call sites use this;
+/// the audit utility uses the full match.
+pub fn extract_date_from_filename(filename: &str) -> Option<FilenameDate> {
+    match_filename_date(filename).map(|m| m.date)
+}
+
 /// Sanity gate for filename-derived timestamps. Real photo capture dates
 /// live in a narrow window; values outside it are almost always sequential
 /// scan IDs (`000227580005.jpg` → 1970) or arbitrary numeric suffixes
@@ -782,6 +830,32 @@ mod tests {
         assert_eq!(date_time.timestamp(), 1383929602);
     }
 
+    // Pattern-name seam (ticket #117): the audit utility prints which
+    // pattern fired, so every class pattern must report a stable name.
+    #[test]
+    fn test_match_filename_date_reports_pattern_names() {
+        let cases: &[(&str, &str)] = &[
+            ("Screenshot_2014-06-01-20-44-50.png", "screenshot"),
+            ("Screenshot_20140601_204450.png", "screenshot_compact"),
+            ("2015-01-09_02-15-15.jpg", "dash"),
+            ("2015-01-09-021515.jpg", "dash_compact"),
+            ("20140927101712.jpg", "compact"),
+            ("att_1422489664680106.jpeg", "unix_timestamp"),
+            ("1401638400.jpeg", "unix_timestamp"),
+        ];
+        for (name, pattern) in cases {
+            let match_ =
+                match_filename_date(name).unwrap_or_else(|| panic!("no match for {}", name));
+            assert_eq!(match_.pattern, *pattern, "pattern name for {}", name);
+        }
+    }
+
+    #[test]
+    fn test_match_filename_date_out_of_class_is_none() {
+        assert_eq!(match_filename_date("IMG_12345.jpg"), None);
+        assert_eq!(match_filename_date("000227580005.jpg"), None);
+    }
+
     // The obsolete `test_memory_date_priority_*` tests covered the old
     // request-time waterfall in `get_memory_date_with_priority`. Their
     // replacement lives in `crate::date_resolver::tests` (resolver