take_blobs(indices=...) fails with _rowaddr schema collision on multi-fragment tables

Version: Lance 3.0.0 (via pylance)

Description:

Calling LanceDataset.take_blobs(blob_column, indices=[...]) on a multi-fragment table with a blob column fails with a schema error. Lance internally tries to append a _rowaddr column, but the blob column's struct schema already contains a _rowaddr field, causing a collision.

Reproduction:

import lance
import random

ds = lance.dataset("path/to/multi_fragment_table.lance")

indices = sorted(random.sample(range(ds.count_rows()), 10))

# This fails:
blobs = ds.take_blobs("my_blob_column", indices=indices)

Error:

RuntimeError: LanceError(Arrow): Schema error: Can not append column _rowaddr on schema:
Schema { fields: [
  Field { name: "my_blob_column", data_type: Struct([
    Field { name: "position", data_type: UInt64, nullable: true },
    Field { name: "size", data_type: UInt64, nullable: true }
  ]), metadata: {"lance-encoding:blob": "true"} },
  Field { name: "_rowaddr", data_type: UInt64, nullable: true }
], metadata: {} },
/home/runner/work/lance/lance/rust/lance/src/dataset/take.rs:384:21

Root cause:

The indices code path in take.rs:384 attempts to add a _rowaddr column to resolve logical indices into physical row addresses. However, the internal blob schema already includes _rowaddr as a field, causing a duplicate column error.

Workaround:

Use ids= instead of indices= with properly encoded row IDs (fragment_id << 32 | row_offset):

fragments = ds.get_fragments()
frag = fragments[0]
offsets = random.sample(range(frag.count_rows()), 10)
row_ids = [(frag.fragment_id << 32) | offset for offset in offsets]
blobs = ds.take_blobs("my_blob_column", ids=row_ids)  # works

Additional notes:

• take_blobs(ids=...) with flat sequential integers (not encoded row IDs) also fails — the IDs must encode the fragment ID in the upper 32 bits.
• LanceDataset.take() does not support with_row_id or with_row_address kwargs, so there's no convenient way to map logical indices to row IDs for use with take_blobs(ids=...).
• The indices path is the only ergonomic API for random access to blobs by logical position, and it's broken on any table with more than one fragment.

Environment:

• Lance 3.0.0
• Python 3.12

Start with the indices path in lance/src/dataset/take.rs at line 384 and reproduce the failure using a multi-fragment table with a blob column and take_blobs(indices=...). Compare it with the working encoded ids= path; done means logical-index blob retrieval no longer hits the _rowaddr schema collision, with the relevant behavior covered by a regression test.