Add configurable V1 blob encoding for large payload columns

[beinan]

Summary

• Add blob_columns: HashSet<String> to ContextStoreOptions to let users opt columns into Lance V1 blob encoding
• Valid columns: text_payload and binary_payload — blob-encoded columns store data in out-of-line buffers for efficient storage of large/unpredictable content
• batch_to_records() auto-detects column types from the batch schema, preserving backward compatibility with existing non-blob datasets
• Python bindings expose blob_columns parameter on Context.create()

Usage

// Rust
let options = ContextStoreOptions {
    blob_columns: HashSet::from(["binary_payload".into(), "text_payload".into()]),
    ..Default::default()
};
let store = ContextStore::open_with_options(uri, options).await?;

# Python
ctx = Context.create("./store", blob_columns=["binary_payload", "text_payload"])

Test plan

• test_blob_binary_payload — roundtrip with blob-encoded binary_payload
• test_blob_text_payload — roundtrip with blob-encoded text_payload (LargeUtf8 → LargeBinary)
• test_blob_both_columns — both columns blob-encoded simultaneously
• test_no_blob_default — default (no blob) schema unchanged
• test_blob_schema_metadata — verifies lance-encoding:blob metadata on fields
• test_blob_invalid_column_name — rejects unknown column names
• test_batch_to_records_autodetects_text_type — auto-detection works for both LargeUtf8 and LargeBinary batches
• All 15 tests pass (cargo test -p lance-context-core)
• Python bindings compile (cargo check -p lance-context-python)

🤖 Generated with Claude Code