feat: Add Apache Lance as a new accepted sink format

[malon64]

Summary

Add Apache Lance as a new accepted sink format alongside delta, iceberg, and parquet.

Motivation

Lance is a modern columnar format designed for ML and analytical workloads, built Arrow-first from the ground up. It features versioned datasets (snapshots), native vector index support, and compatibility with S3/GCS/ADLS — making it a relevant target for data platforms that feed ML pipelines or vector search engines.

Technical Feasibility

The lance Rust crate is production-ready and integrates naturally with the existing Floe stack:

• Arrow-native: Dataset::write() accepts RecordBatch directly — the existing Arrow conversion layer in Floe is immediately reusable.
• Versioned snapshots: Lance has a snapshot model similar to Iceberg, meaning Append and Overwrite semantics map cleanly.
• Cloud storage: Native support for S3, GCS, and ADLS via object_store — consistent with Floe's Target enum.
• No Target enum changes needed: Lance datasets are directory-based (like Delta), so SinkTarget.path maps directly.

Integration complexity is low — the pattern mirrors the existing Iceberg adapter.

Proposed Implementation

1. New adapter

Create crates/floe-core/src/io/write/lance.rs implementing AcceptedSinkAdapter:

use lance::dataset::{Dataset, WriteMode, WriteParams};

struct LanceAcceptedAdapter;

impl AcceptedSinkAdapter for LanceAcceptedAdapter {
    fn write_accepted(&self, target, df, mode, ...) -> FloeResult<AcceptedWriteOutput> {
        let batch = dataframe_to_record_batch(df)?;
        let params = WriteParams {
            mode: match mode {
                WriteMode::Append => lance::dataset::WriteMode::Append,
                WriteMode::Overwrite => lance::dataset::WriteMode::Overwrite,
                _ => return Err(...),
            },
            ..Default::default()
        };
        Dataset::write(stream_of(batch), &uri, Some(params)).await?;
    }
}

2. Dispatch registration

// io/format.rs
"lance" => Ok(io::write::lance::lance_accepted_adapter()),

3. User config

sink:
  accepted:
    format: lance
    path: s3://my-bucket/warehouse/my_dataset.lance

Write Modes

Mode	Support
append	✅ Native (WriteMode::Append)
overwrite	✅ Native (WriteMode::Overwrite)
merge_scd1	❌ Out of scope (no row-level update API yet)
merge_scd2	❌ Out of scope

Out of Scope (for now)

• Vector index creation post-write
• SCD merge operations
• Lance compaction / dataset optimization hooks