repo: split annotated tags out from total tag count in structure

Mirror what git survey already reports: lightweight tags
(pointing straight at a commit/tree/blob) and annotated tags
(pointing at an OBJ_TAG that is itself stored as a separate
object) are different things in many monorepo contexts, and one
of the differences git survey users routinely care about. Add
an annotated_tags counter to struct ref_stats, populate it in
count_references() by peeking at the ref OID's object type, and
expose it as a sub-row under Tags in the table output and as
references.tags.annotated.count in the machine-readable formats.

Step toward pivoting the standalone git survey command onto
git repo structure; this fills the first of the four feature
gaps documented in the assessment.

Tests in t1901 widened to assert the new row and key.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>