Add UpdatableThetaSketch.compactTrimmed() to compact and reduce to k in one pass

An update sketch retains more than the nominal size k between rebuilds. Callers
who need a result bounded by k currently have to call rebuild(), which mutates
the sketch and rebuilds its hash table, before compact().

This adds two concrete methods on UpdatableThetaSketch:

    public CompactThetaSketch compactTrimmed()
    public CompactThetaSketch compactTrimmed(boolean dstOrdered, MemorySegment dstWSeg)

Both are concrete on the base, so none of the subclasses change. A separate
method rather than a trim flag on compact(): Java has no default parameter
values, so a flag would only ever be passed true, and compact(dstOrdered,
MemorySegment) is abstract with seven overriders, so its signature cannot grow
a parameter in any case.

The implementation gathers the valid entries into a dense array, since the
method must not modify the sketch and QuickSelect.select permutes whatever
array it is given, then selects 0-based index k. That is the same hash value
the (k + 1) 1-based pivot yields in rebuild(), so the two cannot drift apart,
and a test asserts the result serializes byte-for-byte identically to
rebuild() + compact().

Because it never mutates, this also works on a read-only sketch, where
rebuild() throws SketchesReadOnlyException.

The Alpha family is excluded and throws UnsupportedOperationException: it
maintains theta by its own discipline and never needs reducing to k. The guard
is a family denylist rather than a QuickSelect allowlist, because Theta's
family names are historically fragmented and an allowlist could wrongly reject
a legitimate member.

Trimming stays an explicit opt-in because it is lossy. Relative error scales
with 1 / sqrt(retained), so discarding entries always widens the confidence
bounds, and a sketch in exact mode that retains more than k loses exactness and
is returned in estimation mode. Both effects are documented and pinned by tests.

This is the Java counterpart of apache/datasketches-cpp#524.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017EDHa7UhfW4eSj5L82panJ