feat(dates): filename date class — per-pattern conventions + filename-over-EXIF priority (#116)

The filename date class is now first-class (ADR-0001):

- memories::extract_date_from_filename returns FilenameDate:
  Wall(NaiveDateTime) for the screenshot/dash/compact patterns,
  Utc(DateTime<Utc>) for the unix-timestamp patterns. No patterns
  added or removed; every existing pattern carries its convention.
- Waterfall inversion in both resolve_date_taken and
  resolve_dates_batch: an in-class name is the original date of
  record and resolves BEFORE EXIF / container dates are consulted
  (shared filename_class_resolved step; in-class files skip
  exiftool entirely). Wall patterns run the offset cascade over the
  kamadak prior's evidence (empty evidence -> server-zone
  assumption, so a class file always resolves); timestamp patterns
  are true instants (tz_source utc_direct, no offset).
- Legacy date_taken stays byte-identical: wall-as-UTC for wall
  patterns, epoch for timestamp patterns, via
  FilenameDate::legacy_datetime / legacy_timestamp at the seven
  consumer call sites.
- Out-of-class files keep the global EXIF-wins waterfall unchanged.
- CLAUDE.md waterfall section rewritten (step 0 = filename class;
  the EXIF-beats-filename note becomes out-of-class-only).

Tests: five new waterfall-seam tests (wall no-evidence ->
assumed_server; wall outranks EXIF with offset-tag evidence;
wall GPS evidence; timestamp utc_direct; in-class video name
resolves without exiftool) plus enum-shape assertions in every
extractor test. Code-reviewed on both axes (standards: no
violations, 3 of 4 P2 notes applied; spec: all acceptance
criteria verified, no scope creep).

716 integration + 639 lib tests pass; fmt + clippy clean.

src/files.rs

@@ -18,7 +18,7 @@ use crate::data::{
 use crate::database::ExifDao;
 use crate::file_types;
 use crate::geo::{gps_bounding_box, haversine_distance};
-use crate::memories::extract_date_from_filename;
+use crate::memories::{FilenameDate, extract_date_from_filename};
 use crate::thumbnails::create_thumbnails;
 use crate::utils::earliest_fs_time;
 use actix_web::web::Data;
@@ -145,7 +145,9 @@ fn in_memory_date_sort(
             let date_taken = exif_map
                 .get(&(f.file_name.clone(), lib_id))
                 .copied()
-                .or_else(|| extract_date_from_filename(&f.file_name).map(|dt| dt.timestamp()))
+                .or_else(|| {
+                    extract_date_from_filename(&f.file_name).map(FilenameDate::legacy_timestamp)
+                })
                 .or_else(|| {
                     lib_roots.get(&lib_id).and_then(|root| {
                         let full_path = Path::new(root).join(&f.file_name);