Add Seek and RecordIndex for random record access

[chrisrichards]

Summary

Reading has been strictly forward-only — no way to jump to a record by index, and no way to tell which record was just read. This adds:

• DbfTable.Seek(int recordIndex) — positions the table at the given zero-based record (dataStart + recordIndex * RecordLength). Also exposed as DbfDataReader.Seek. Negative indexes throw ArgumentOutOfRangeException; non-seekable streams throw NotSupportedException; seeking past the last record is allowed and simply makes the next Read() return false (deliberately not validated against Header.RecordCount, which is unreliable in real-world files).
• DbfRecord.RecordIndex — zero-based index of the record most recently read, -1 before any read. Also exposed as DbfDataReader.RecordIndex. The value is derived from the stream position rather than a counter, so it stays correct across seeks and across multiple DbfRecords reading interleaved from one table. With SkipDeletedRecords, it reports the file index of the record actually surfaced.
• The offset math accounts for streams where the DBF content starts at a non-zero position: the table captures the stream position at construction and bases all record offsets on it.

Design notes

• Zero-based throughout, so Seek(record.RecordIndex) + Read re-reads the current record — one consistent convention. (xBase record numbers are traditionally 1-based; a future CDX index integration will convert in one documented place.)
• This is the groundwork step for CDX index support from the fork analysis: index searches yield record numbers that need Seek to become rows.

Test plan

New SeekTests (8 tests): sequential RecordIndex tracking (-1 → 0..13), seek-then-read matches sequential values, rewind after full scan, seek past end reads nothing, negative index throws, non-seekable stream throws, DBF embedded at a non-zero stream offset seeks correctly, and the DbfDataReader-level Seek/RecordIndex path. Full suite: 112 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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