crl-release-26.1: valsep: account for blob-file compression in split-decision size estimate

[RaduBerinde]

During a compaction, the size of an output sstable is estimated as keys
are written in order to decide when to split into a new table. When the
compaction preserves existing blob references, the estimate added the
sum of the uncompressed value sizes of the preserved references.

Because blob files are compressed on disk, this overestimated the disk
space the references contribute for compressible data, splitting output
tables earlier than intended and producing smaller-than-target files.
The final stored TableMetadata already accounted for compression via
BlobReference.EstimatedPhysicalSize; only the in-flight estimate used
uncompressed sizes.

ValueSeparator.EstimatedReferenceSize now scales each preserved
reference by its source blob file's compression ratio, using the same
computation as manifest.MakeBlobReference, so the in-flight estimate
converges to the size the table will ultimately report. The source
PhysicalBlobFile is resolved once when a preserved reference is first
created (rather than at FinishOutput) and carried on pendingReference;
the preserved boolean is replaced by a preserved() method that
reports phys \!= nil.

The shared scaling computation is centralized in a new
PhysicalBlobFile.EstimatedReferencePhysicalSize method that performs
the multiplication in 128-bit precision (bits.Mul64/bits.Div64) to
avoid overflow for large blob files.

Co-Authored-By: roachdev-claude roachdev-claude-bot@cockroachlabs.com

[cockroach-teamcity]

This change is