…_reuse (Phase 2a)

Phase 2a of the migration plan in
``juniper-ml/notes/observability/REGISTER_OR_REUSE_HELPER_DESIGN_2026-05-05.md``.
Drops the inline ``_get_or_create`` helper added in PR #87 and calls
the canonical ``juniper_observability.register_or_reuse`` shipped in
juniper-observability ``0.2.0``.

### What changes

- ``juniper_data/api/observability.py``: ``_ensure_dataset_metrics``
  now imports ``register_or_reuse`` from ``juniper_observability``
  (lazy, inside the ``if _dataset_metrics is None`` branch) and uses
  it for all four collectors (Counter ×2 + Histogram + Gauge). The
  inline ``_get_or_create`` closure and the manual
  ``REGISTRY._names_to_collectors`` lookup are gone.
- ``pyproject.toml``: ``juniper-observability>=0.1.1`` →
  ``juniper-observability>=0.2.0``. The new helper is the reason for
  the bump; existing 0.1.1 callers' behaviour is unchanged.

### Drive-by fix: storage circular import

PR #92 (``audit-fixup: wire juniper_data_datasets_cached gauge to
cache layer``) added ``from juniper_data.api.observability import
set_datasets_cached`` at the top of
``juniper_data/storage/cached.py``. That triggers the import chain:

    juniper_data.storage.__init__
      → cached.py
      → juniper_data.api.observability
      → juniper_data.api.__init__
      → app.py
      → juniper_data.storage  (← still initialising!)
      → ImportError: cannot import name 'LocalFSDatasetStore'

…which broke ``pytest --collect-only`` on origin/main as of
2026-05-06: 31 collection errors covering every test that imports
``juniper_data.storage`` directly or transitively. ``--collect-only``
amplifies the breakage, but a normal full pytest run also hits the
same path during fixture collection.

Fix: defer the ``set_datasets_cached`` import to inside
``CachedDatasetStore._emit_cached_count`` so the cycle never fires
at module-import time. The function is best-effort with a
``except Exception`` swallowing failures, so a deferred import that
raises ``ImportError`` (e.g. during a ``CachedDatasetStore``
instantiation in a non-API context) gets logged at DEBUG and skipped
the same way any other observability failure does.

### Verification

Full juniper-data suite under JuniperData env: **950 passed**
(was ``31 errors during collection`` on origin/main pre-fix).

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