date_resolver: canonical date_taken waterfall with exiftool fallback

New module that consolidates the four-step ingest waterfall:
kamadak-exif (already in process via the caller's prior result) →
exiftool fallback → filename regex → earliest_fs_time. Each step is
tagged with a `DateSource` so the caller can persist provenance.

The exiftool fallback is what makes videos and MakerNote-hosted dates
land at all — kamadak-exif can't read QuickTime/MP4 or Nikon-style
sub-IFDs. Single-file mode shells out per call; batch mode pipes paths
on stdin via `-@ -` and fans the result through one subprocess so the
upcoming per-tick drain doesn't pay startup cost per row. The
`exiftool` PATH check is cached in a `OnceLock` to keep the drain
short-circuited on deploys without exiftool installed.

`SubSecDateTimeOriginal` and `ContentCreateDate` are pulled alongside
the standard tags to capture iPhone's sub-second precision and Apple's
preferred capture-time tag respectively. `FileModifyDate` is
deliberately *not* in the tag list — it's a filesystem-derived value
the resolver already covers via the `fs_time` step, and pulling it
through exiftool would mask "no real EXIF date" with a misleading
`source = exiftool` row.

Module is registered in both `lib.rs` and `main.rs` (sibling-module
pattern the rest of the bin uses); no callers wired in yet — that
lands in the next commit. Comes with 9 unit tests covering JSON
parsing edge cases, source-priority short-circuiting, and the
fs_time-when-no-exif path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib.rs

@@ -10,6 +10,7 @@ pub mod cleanup;
 pub mod content_hash;
 pub mod data;
 pub mod database;
+pub mod date_resolver;
 pub mod duplicates;
 pub mod error;
 pub mod exif;