Use sync-first TDS cursor reads in ODBC - #307

Description

Layer 4 (top) of a separate experimental performance stack.

Updates the existing row-at-a-time / column-at-a-time ODBC paths to use layer 3's explicit sync-first TDS cursor APIs:

• SQLFetch calls try_next_row_cursor before the async fallback.
• SQLGetData calls try_read_row_column before the async fallback.
• CursorPoll::Ready bypasses Tokio; CursorPoll::Pending invokes the existing async method exactly once.

All existing client ownership, DBC/statement locking, diagnostics, INFO handling, active-statement state, PLP behavior, timeout/cancellation behavior, and error recovery remain unchanged. This does not batch, prefetch, or materialize whole rows.

Stack parent: #306

Linux benchmark

SQL Server 2022 in Podman, unixODBC, Encrypt=No, TrustServerCertificate=Yes, CPUs 8-15. Each process used two warmups and seven measured passes over 200,000 rows with SQLExecDirect + SQLFetch + four sequential SQLGetData calls (INT, BIGINT, NVARCHAR, BIT). Runs were counterbalanced parent/candidate/candidate/parent.

• Throughput: +203.87%
• Latency: -67.09%
• Paired process gains: +206.65% and +198.50%
• Identical checksum on every run
• Approximately 39.9% of the previously measured patched raw-TDS throughput (2.528M rows/s)

Raw result data is retained in the authoring session artifacts.

Correctness coverage

The resolve_cursor_poll tests prove:

• Ready skips fallback entirely.
• Pending invokes fallback exactly once.
• A sync-attempt error does not invoke fallback.

The full loadable-driver e2e suite passes against SQL Server, covering SQLFetch, SQLGetData, PLP, result navigation, transactions, and recovery through unixODBC.

Validation

• cargo fmt --all -- --check
• cargo nextest run -p mssql-odbc --no-fail-fast — 551 passed
• cargo clippy -p mssql-odbc --frozen --all-features --all-targets -- -D warnings
• mssql-odbc/tests/e2e/run_e2e.sh --release --driver=... — 19/19 test binaries passed

Limitations

The sync-first coverage is defined by layer 3. PLP, encrypted, skipped, unsupported, incomplete, and control-token paths continue through the existing async implementation.

Related Issues

Related to #306.

Checklist

• Formatting passes
• Clippy passes with warnings denied
• Unit and loadable-driver e2e tests pass
• Strict per-row/per-column streaming is preserved
• No batching, prefetch, or whole-row materialization