feat(python): blob v2 fetch API (#3578)

Python bindings for blob v2 read on **local** tables. Rust read APIs
landed in #3562.

This PR wires `fetch_blob_files`, `fetch_blobs`, v2
query/`to_pandas(blob_mode="bytes")`, and hidden `_rowid` metadata so
`fetch_*` works from query hits without exposing `_rowid` in the column
list.

**Cloud:** `RemoteTable.fetch_blobs` / `fetch_blob_files` raise
`NotImplementedError` until Phalanx ships the server route (separate
track; not blocking local merge).

### Primary path: lazy file handles

```python
table = db.create_table("videos", schema=pa.schema([
    pa.field("id", pa.int64()),
    lancedb.blob("video"),
]))
table.add([{"id": 1, "video": open("clip.mp4", "rb").read()}])

hits = table.search().select(["id", "video"]).to_arrow()
handle = table.fetch_blob_files("video", hits)[0]

# seek + partial read — PyAV / decoders can use the handle
handle.seek(frame_offset)
chunk = handle.read_range(0, 65536)
```

`BlobFile` exposes `seek`, `read`, `read_range`, `read_up_to`, and works
with `BufferedReader`.

### When you want full bytes

```python
blobs = table.fetch_blobs("video", hits)  # eager materialize, null-aligned
df = table.to_pandas(blob_mode="bytes")   # descriptors → bytes in pandas
```

### `_rowid` (join key, not user `id`)

Fetch needs Lance row ids. For v2 blob queries we auto-inject `_rowid`,
stash it in Arrow schema metadata on `to_arrow()`, and drop the visible
column unless you pass `.with_row_id(True)`.

v1 legacy blobs (`lance-encoding:blob`) unchanged; fetch on v1 raises
the migration error.

## Test plan

- [x] `./scripts/test-blob.sh python` (105 passed in worktree)
- [x] `fetch_blob_files` lazy read, seek, partial read, null alignment,
cross-fragment dups
- [x] hybrid query → `fetch_blobs` / `fetch_blob_files`
- [ ] Will re-review after seek/`BlobFile` commit (`d77ab1a6`)

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

python/src/lib.rs

@@ -16,7 +16,7 @@ use query::{FTSQuery, HybridQuery, Query, VectorQuery};
 use session::Session;
 use table::{
     AddColumnsResult, AddResult, AlterColumnsResult, DeleteResult, DropColumnsResult, LsmWriteSpec,
-    MergeResult, Table, UpdateFieldMetadataResult, UpdateResult,
+    MergeResult, PyBlobFile, Table, UpdateFieldMetadataResult, UpdateResult,
 };
 
 pub mod arrow;
@@ -44,6 +44,7 @@ pub fn _lancedb(_py: Python, m: &Bound<'_, PyModule>) -> PyResult<()> {
     m.add_class::<Connection>()?;
     m.add_class::<Session>()?;
     m.add_class::<Table>()?;
+    m.add_class::<PyBlobFile>()?;
     m.add_class::<IndexConfig>()?;
     m.add_class::<Query>()?;
     m.add_class::<FTSQuery>()?;