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.

CLAUDE.md

@@ -374,6 +374,18 @@ per-tick drain can re-resolve weak entries when better tools become
 available, and so the UI/debug surface can answer "why did this photo
 land on this date?". Order:
 
+0. **`filename` date class (ADR-0001, ticket #116)** —
+   `extract_date_from_filename` in `memories.rs` returns a
+   `FilenameDate`: wall-clock patterns (screenshot, dash, compact) or
+   unix-timestamp patterns. A file whose name matches **any** pattern is
+   *in the class*: the filename is the original date of record and wins
+   *before* EXIF / container dates are consulted, inverting the global
+   EXIF-wins rule for exactly those files (a renamed screenshot or a
+   chat-export timestamp is the only honest date left). Wall patterns
+   run the offset cascade over the file's own `OffsetTime*` / GPS
+   evidence (empty evidence → server-zone assumption, so a class file
+   always resolves); timestamp patterns are true instants outright
+   (`utc_direct`, no offset). In-class files never hit exiftool.
 1. **`exif`** — kamadak-exif `DateTime` / `DateTimeOriginal`. Fast,
    in-process, image-only.
 2. **`exiftool`** — shell-out fallback for tags kamadak can't reach:
@@ -395,17 +407,15 @@ land on this date?". Order:
    old server-local `-d %s` interpretation; the new-contract
    `date_utc` / `camera_offset_minutes` / `date_taken_tz_source`
    columns carry the true instant.
-3. **`filename`** — `extract_date_from_filename` in `memories.rs`
-   matches screenshot, chat-export, and timestamp-named patterns.
-4. **`fs_time`** — `earliest_fs_time(metadata)` (earlier of created /
-   modified). Last resort.
+3. **`fs_time`** — `earliest_fs_time(metadata)` (earlier of created /
+   modified). Last resort. (The filename step sits at step 0 for
+   in-class names; an out-of-class name matches nothing here.)
 
-Notable behavior change vs. the pre-2026-05 request-time logic:
-**EXIF beats filename when both are present.** A photo named
-`Screenshot_2014-06-01.png` whose EXIF `DateTime` is 2021 now appears
-under 2021, not 2014 — on the theory that EXIF is more reliable than
-import-named filenames. The reverse case (no EXIF, filename has a
-date) is unchanged.
+**Out-of-class files keep EXIF-wins.** A photo named `IMG_1234.jpg`
+whose EXIF `DateTime` is 2021 appears under 2021 — EXIF
+`DateTimeOriginal` is more reliable than an import-time filename. The
+filename class (step 0) is the documented exception where the name is
+the original date itself.
 
 The `backfill_missing_date_taken` drain (`src/backfill.rs`) runs every
 watcher tick alongside `backfill_unhashed_backlog` (also `src/backfill.rs`). It loads up to