Promote single canonical identify-stream EXIF parser into metadata-cache (8r0)

The `identify -verbose` EXIF-line regex and its array-fill loop were
duplicated in three places that had already drifted: album-metadata's
photo_exif_details_html and _photo_exif_values_to (exif: only) and
stats-aggregate's _stats_parse_identify_stream (exif: plus a native
Geometry -> __geometry path).

Promote one canonical parser, photo_exif_values_to, into
metadata-cache.source.sh next to its sibling cache primitive
cached_photo_identify_output. It reads an identify stream from stdin and
fills a nameref associative array; it is a strict superset of all three
former sites (bare exif: tag keys plus the synthetic __geometry key).

- stats accumulate_photo_stats now calls photo_exif_values_to (stdin);
  _stats_parse_identify_stream is removed.
- album _photo_exif_values_to is a thin wrapper that pipes
  cached_photo_identify_output through the canonical parser.
- album photo_exif_details_html consumes the same parser and skips the
  __geometry key it does not display.

metadata-cache is sourced before both consumers in LIB_SOURCES, so the
canonical parser is available at use time. Regenerated bin/shuriken via
`just build`. Added test_shared_identify_parser_returns_exif_and_geometry
asserting one parse yields both an exif: key and __geometry.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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