Add writeExistingDataManifest for manifest rewrites

[ryo-rm]

Depends on #38 — that fix is the first commit here.

It would be useful to be able to run the standard "rewrite manifests"
maintenance operation with icebird: merging many small fast-append manifests
into one, without touching the data files. A writer that commits frequently
accumulates manifests quickly, and scans slow down with the count.

Most of it is already reachable — icebergManifests, buildSnapshotUpdate
with skipPriorManifestPaths, buildPartitionSummaries. The gap is the write:
carried-over files need status EXISTING and explicit per-entry snapshot_id,
sequence_number and file_sequence_number, while writeDataManifest
hard-codes ADDED and leaves the sequence numbers null for inheritance — which
would move those files to the rewriting snapshot's sequence number and change
which delete files apply to them.

So this adds writeExistingDataManifest, mirroring writeExistingDeleteManifest.
Exporting manifestEntrySchema would work too, though it pins the Avro schema
builder's signature as public API. Happy to go whichever way you prefer.

Rewrite manifests built on this

The call site — the rest is assembling the manifest-list entry and committing,
which buildSnapshotUpdate and fileCatalogCommit already cover:

const entries = (await icebergManifests({ metadata, resolver }))
  .filter(l => dataManifestPaths.has(l.url))
  .flatMap(l => l.entries)
  .filter(e => e.status !== 2)

const writer = resolver.writer(manifestPath)
await writeExistingDataManifest({ writer, schema, partitionSpec, entries, formatVersion })

Run against a v2 table with four data manifests and one position delete file,
merging the data manifests into one and leaving the delete manifest alone:

data manifests before: 4 | delete manifests kept: 1
merged manifest: min_sequence_number=1 sequence_number=6
manifests after: 1 data + 1 delete

rows before rewrite: 0,1,3,4,5,99
rows after rewrite:  0,1,3,4,5,99

Building the same merge on writeDataManifest instead resurrects the deleted
row: the carried-over files take the rewriting snapshot's sequence number, so
the delete file no longer applies to them.

merged manifest: min_sequence_number=6 sequence_number=6
rows after rewrite:  0,1,2,3,4,5,99