[core] Spell out what a negative partition statistic means

[sundapeng]

Purpose

PartitionStatistics says only that its fields "may be negative, indicating that some data has been
removed". That covers one of the two planes the class is read on, and consumers have been getting the
other one wrong.

• Delta plane — what a commit changed. A negative value is a decrement the server adds to what it
  holds. That is the existing meaning and nothing here changes it.
• Observation plane — what listPartitions returns for a partition as it stands. A negative value
  means nobody ever reported that field, and 0 means an exact zero.

Conflating them is not cosmetic. A consumer that reads unknown as zero plans against an empty
partition that may hold a billion rows; one that does arithmetic on it gets a number that is wrong
rather than missing. This has already produced a silent wrong answer in a sibling project, where
COUNT(*) was answered from a placeholder row count and returned zero for a partition full of data
(apache/paimon-rust#624).

So: the plane is named in the javadoc, unknown gets a name (UNKNOWN, with isKnown() to test it
rather than each caller comparing against -1), and unknown is documented as per field — a
reporter that only knows the file count leaves the record count unknown and fills the rest.

Why the fields stay primitive

Boxing them to express unknown as null would be a breaking change to a @Public class, and the
encoding above needs no new type.

Behaviour

Zero change. UNKNOWN is a name for a value already in use. FileSystemSplitEnumerator now says
PartitionStatistics.UNKNOWN where it said -1 — the same value under its own name, because
discovering partitions by listing directories measures nothing about what is inside them, which is
what unknown already meant there.

Context

This is the first of a stack that lets a catalog-managed format table report partition statistics; a
format table has no snapshot, so the channel a table snapshot uses does not exist for it. This PR is
worth having on its own regardless of the rest of that stack: the ambiguity it removes has already
cost one silent wrong answer.

API and Format

PartitionStatistics gains UNKNOWN, UNKNOWN_TOTAL_BUCKETS, unknown(spec) and
isKnown(long). Additive only; no existing signature changes. No format change.

Documentation

The contract now lives in the class javadoc, which is where a consumer looks.