perf:Add additional sharding benchmarks

[mkitti]

Summary

Added benchmarks for monitoring Morton order computation in sharded arrays. These benchmarks help assess the impact of Morton order optimizations in the context of I/O operations.

Benchmarks Added

• test_sharded_morton_indexing - Sharded array indexing with power-of-2 chunks per shard
• test_sharded_morton_indexing_large - Large shard with 32^3 = 32,768 chunks
• test_sharded_morton_single_chunk - Reading a single chunk from a large shard
• test_morton_order_iter - Direct benchmark of morton_order_iter (no I/O)
• test_sharded_morton_write_single_chunk - Writing a single chunk to a large shard (best end-to-end test)

Benchmark Results

Single Chunk Write (Best End-to-End Test)

Writing a single 1x1x1 chunk to a shard with 32^3 = 32,768 chunks:

Branch	Mean Time	Improvement
Main (no optimization)	425ms	-
Optimized (PR #3708)	261ms	164ms (39% faster)

Morton Order Computation (Micro-benchmark)

Direct morton_order_iter benchmark without I/O:

Shape	Main Branch	Optimized	Speedup
(8, 8, 8)	2.73ms	0.85ms	3.2x
(16, 16, 16)	25.53ms	6.31ms	4.0x
(32, 32, 32)	229.25ms	51.31ms	4.5x

Profiling Analysis

Profile of single chunk write benchmark showing where time is spent:

Main Branch (977ms total)

Function	Time	Calls	% of Total
decode_morton (scalar)	289ms	32,768	30%
get_chunk_slice	104ms	32,768	11%
_localize_chunk	103ms	32,768	11%
_morton_order	99ms	1	10%
Generator expressions	94ms	262k	10%
all() / len()	87ms	263k	9%

Optimized Branch (456ms total)

Function	Time	Calls	% of Total
get_chunk_slice	110ms	32,768	24%
_localize_chunk	105ms	32,768	23%
_morton_order	66ms	1	14%
Generator expressions	38ms	131k	8%
decode_morton_vectorized	9ms	1	2%

Key Optimization Wins

1. Vectorized decoding: Eliminates 32,768 scalar decode_morton calls (289ms → 9ms)
2. Reduced bounds checking: Hypercube optimization eliminates all() checks for in-bounds coordinates
3. Fewer function calls: 1.1M calls reduced to 299k calls

Remaining Optimization Opportunity

get_chunk_slice and _localize_chunk are called 32,768 times even when writing a single chunk due to line 508 in sharding.py:

shard_dict = {k: shard_reader.get(k) for k in morton_order_iter(chunks_per_shard)}

This builds a dict of ALL chunks before writing. Optimizing this read-modify-write pattern could save an additional ~215ms.

Checklist

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.md
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[mkitti]

If we wanted to minimize this pull request, I would reduce it to just "test_sharded_morton_write_single_chunk".

[mkitti]

@d-v-b merge or add benchmark label, please.