perf(format): parallel cached decode and fewer copies on full reads

Same-moment A/B on a 64 MB f64 dataset: chunked+deflate 110 -> 69 ms, chunked
72 -> 60 ms, contiguous 56 -> 30 ms.

- read_chunked_data_cached — the path the facade uses — decompressed chunks
  one at a time; only the uncached reader was parallel. Cache misses are now
  decoded in bounded batches (128), in parallel with the `parallel` feature.
- Every chunk was pushed into the 16 MiB chunk cache, which a larger dataset
  just churns (insert, evict moments later). Chunks are cached only when the
  whole dataset fits (new ChunkCache::max_bytes).
- Unfiltered chunks went file -> Vec -> aligned cache buffer -> output. They
  are copied straight from the file bytes.
- The facade's typed reads convert a contiguous dataset straight from the
  borrowed file bytes instead of copying it into a Vec first.
- The native little-endian fast paths allocated vec![0; n] and then overwrote
  it; they now fill an uninitialised buffer in one copy (native_le_to_vec).
  alloc_output requests zeroed memory from the allocator instead of reserving
  and filling.

The unit test that expected unfiltered chunks to land in the decompressed
cache now asserts the new design (index reused, cache not involved).

Co-Authored-By: Claude Fable 5.1 <[email protected]>

CHANGELOG.md

@@ -11,6 +11,13 @@
   2.7 ms, one column 5.2 ms. Results are identical to the full-read path
   (equivalence-tested over random hyperslabs and point lists, ranks 1-3,
   contiguous / chunked / deflate). New `read_harness` bench binary.
+- **Faster full reads** (same-moment A/B, 64 MB `f64`): chunked + deflate
+  110 -> 69 ms, chunked 72 -> 60 ms, contiguous 56 -> 30 ms. The facade's
+  cached read path now decompresses cache misses in parallel batches (it was
+  sequential; only the uncached reader was parallel) and caches only datasets
+  that fit the chunk cache; unfiltered chunks are copied straight from the file
+  bytes; a contiguous dataset is converted straight from the file bytes; and
+  the native-endian conversions no longer zero a buffer before overwriting it.
 - **Datasets indexed by a version-2 B-tree now read** (layout v4, chunk index
   type 5 — what `libver='latest'` uses for two or more unlimited dimensions;
   previously "unsupported chunked layout"). The four copies of the chunk-index