Add asynchronous record reading

[chrisrichards]

Summary

Adds true async I/O to the read path — until now DbfDataReader inherited DbDataReader's default ReadAsync, which just wraps the synchronous Read() in a completed task.

Design: one buffered async read per record, synchronous parsing. DbfRecord.ReadAsync fills the existing reused record buffer with Stream.ReadAsync(Memory<byte>, CancellationToken) (same partial-read loop as the sync path), then parses via the existing span-based value readers. The parse is shared with the sync path through a private ParseRecord (spans can't live in async methods, which makes the split natural). This is deliberately the opposite of the 2018 fork's approach (per-field async through a hand-ported async BinaryReader) — one awaited read per record instead of one per field, and a single code path for all value parsing.

New API:

• DbfRecord.ReadAsync(Stream, CancellationToken) → ValueTask<bool>
• DbfTable.ReadAsync(DbfRecord, CancellationToken) → ValueTask<bool>
• DbfTable.ReadRecordAsync(CancellationToken) → ValueTask<DbfRecord>
• DbfDataReader.ReadAsync(CancellationToken) — real override with the same skip-deleted loop as Read()

ValueTask internals mean no per-record task allocation when reads complete synchronously from the FileStream buffer (the common case); the Task<bool>-returning override benefits from the cached true/false task optimisation. Cancellation flows through to the underlying stream. EndOfStreamException handling matches the sync path.

Scope notes:

• File opening is unchanged (no FileOptions.Asynchronous by default — with one buffered read per record the handle mode matters little, and it would slow the dominant sync path; callers wanting a fully async handle can pass their own FileStream via the stream constructors).
• Memo values are still resolved synchronously during parsing (small seeked reads); can be made async separately.
• Possible follow-ups: async table open (OpenAsync for header/columns), IAsyncEnumerable<DbfRecord>.

Test plan

New AsyncReadTests (7 tests): full-table async read compared row-by-row against the sync path (dbase_03, and dbase_30 to cover memo resolution inside async reads), RecordIndex tracking, Seek + ReadAsync, ReadRecordAsync, skip-deleted through the reader override (12 of 14 rows), and cancellation (pre-cancelled token throws OperationCanceledException). The existing DbfDbConnection tests already call ExecuteReaderAsync/ReadAsync and now exercise the real override. Full suite: 119 passed, 1 skipped (pre-existing WIP skip).

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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