Adopt zarr-python #4028 primitives: read_regions transport + shard-index occupancy fallback

[espg]

🤖 from Claude

Context

zarr-developers/zarr-python#4028 (in review) adds shards_initialized,
initialized_regions, initialized_chunk_regions, and read_regions. moczarr's
discovery is arithmetic (coverage MOCs, no listings), so these don't replace the MOC
path — they slot in underneath and beside it. Filed now so the flip-list is on record;
blocked on the first zarr-python release carrying #4028.

Integration points

1. read_regions as the within-leaf transport. moczarr computes populated regions
   from coverage metadata with zero store round-trips; read_regions takes
   caller-supplied regions by design and does the concurrent fetch/decode. For sharded
   leaves (the zagg#233 default), feeding MOC-derived regions to read_regions replaces
   per-leaf assembly plumbing with upstream-maintained concurrency, and the sharding
   codec's partial decode already skips empty inner chunks. (Follow-on to the open_hive: shared store handle — O(leaves) store opens must collapse before thousand-leaf stores #5
   store-handle consolidation: same goal, one more layer collapsed.)
2. Degraded-path occupancy from shard indexes — posture decision. Today a corrupt
   coverage-bitmap sidecar raises (the D9 family: degrade, never wrong answers — but
   for sidecars we currently have nothing to degrade to). initialized_chunk_regions
   recovers true inner-chunk occupancy from the shard indexes themselves (one small
   range-read per populated shard, no chunk data). Options:
   • keep raising (sidecar corruption stays loud), or
   • degrade to shard-index introspection with a warning — slower but correct, matching
     the "unusable root MOC → walk" posture.
     This is a semantics choice for @espg; the primitive makes the second option cheap to
     implement either way.
3. Sidecar cross-validation. The bitmap sidecar and the shard index declare the same
   occupancy from different sources (metadata vs. bytes). A test-suite / tools/ check
   that they agree on the fixture store (and optionally an opt-in runtime verify flag)
   turns #4028 into a correctness cross-check on the whole write path, not just a read
   optimization.

Out of scope

• The hive-tree discovery walk (leaf group discovery) — that's store-level listing of
  the digit tree, not array-level occupancy; #4028 doesn't apply.
• prototype for negative caching in StoreCache zarr-developers/zarr-python#4042 (CacheStore negative caching) — no moczarr code
  change; at most a docs mention, since the MOC path never issues the empty reads that
  negative caching remembers.

Gating / acceptance

• zarr-python floor bump to the release carrying #4028 (extra or core dep as
  appropriate).
• Acceptance: sharded-leaf reads route through read_regions; the sidecar/shard-index
  agreement check runs in CI against the fixture store; posture decision from (2)
  recorded here and implemented or explicitly declined.

Cross-refs: #1 (plan: layers), #5 (store-handle consolidation), englacial/zagg#233,
zarr-developers/zarr-python#4028 / #3929 / #4042.