Add sync-first TDS cursor APIs

[Saurabh Singh (saurabh500)]

Description

Layer 3 of a separate experimental performance stack.

Adds explicit non-blocking pull-cursor APIs to TdsClient:

• try_next_row_cursor
• try_read_row_column
• CursorPoll::{Ready, Pending}

The ready path parses already-buffered ROW/NBCROW headers and common unencrypted non-PLP scalar, string, and binary columns without entering Tokio or constructing an async cursor future. A pending attempt consumes neither transport bytes nor cursor state; callers continue with the existing async next_row_cursor / read_row_column methods.

Strict row-at-a-time and column-at-a-time streaming is preserved. This does not batch, prefetch, or materialize whole rows.

Stack parent: #304

Correctness boundaries

• PLP, encrypted, skipped, unsupported, incomplete, and control-token paths remain on the existing async implementation.
• Buffered decoding commits TdsReadBuffer consumption only after a complete value is available.
• Packet-boundary and NBCROW bitmap misses leave partial bytes intact for async continuation.
• Cancellation is deferred to the async path without consuming state.
• Explicit zero-timeout budgets retain ready-first semantics.

Measurement context

A preceding proof-of-concept that synchronously polled cursor futures demonstrated a large opportunity, but depended on executor/waker behavior. This layer replaces that evidence-only mechanism with a cursor-specific, documented TDS state-machine API. Final ODBC throughput is intentionally deferred to the next layer that adopts these APIs.

Related Issues

Related to #304.

Validation

• cargo fmt --all -- --check
• 13 focused sync-cursor, buffered-decoder, packet-boundary, NBCROW, timeout, and cancellation tests
• cargo clippy -p mssql-tds --frozen --all-features --all-targets -- -D warnings
• RUSTFLAGS='--cfg fuzzing' cargo check -p mssql-tds --lib
• Full mssql-tds library suite: 1,681 passed; four pre-existing certificate-fixture tests failed because tests/test_certificates/valid_cert.pem and .der are absent

Checklist

• Formatting passes
• Clippy passes with warnings denied
• Focused tests pass
• Strict per-row/per-column streaming is preserved
• Pending paths are non-destructive