Core: Coalesce nearby blob reads in PuffinReader.readAll

[vishnuprakaz]

PuffinReader.readAll currently issues one seek + read per blob. This resolves
the long-standing TODO from #4537:

Contiguous (or overlapping) blobs are now read in a single request and handed back
as no-copy ByteBuffer views over that region. Since Puffin stats/index blobs are
written contiguously, a readAll over many blobs collapses from N reads to one
the win matters most on object stores, where each read is a round trip.

Approach

This coalesces contiguous blob reads: after sorting by offset, adjacent/overlapping blobs are merged into a single read, and a run is split once it would exceed a max read size.

Also RangeReadable.readVectored(...) was considered as an alternative, but its default implementation doesn't coalesce on S3/Hadoop (only some FileIOs, e.g. GCS analytics, optimize it) and eagerly buffers every range.

Coalescing contiguous reads captures the benefit for real Puffin layouts without reading bytes the caller didn't request, and reads stay lazy (a region is fetched only when iterated).

The max read size is capped at 8 MiB the same value as MANIFEST_TARGET_SIZE, since Puffin blobs are metadata. The cap bounds the extra memory coalescing holds compared to the original per-blob reads (a single blob larger than the cap is still read on its own).

Are these changes tested?

• Existing testReadMetricDataUncompressed / testReadMetricDataCompressedZstd cover
  readAll end-to-end for both codecs (contiguous blobs, nonzero-offset slice).
• New end-to-end readAll test with blobs far apart, asserting both decode across two
  separate reads.
• New unit tests for the grouping logic: contiguous/overlap merge, gap split, region-size
  split, and an oversized single blob kept in its own read.

Are there any user-facing changes?

No