Add S-57 exchange set integrity verification (CATALOG.031 CRC + S-63 seam)

[philliphoff]

Why

The S-57 exchange-set reader parsed each CATALOG.031 CATD record's CRC (the CRCS subfield) but never validated it, so corrupted or tampered cell files went undetected. This implements issue #6: opt-in integrity verification of the files an exchange set references.

What

Adds a non-throwing verifier under EncDotNet.S57/ExchangeSets/ whose API deliberately mirrors the sibling EncDotNet.S100 exchange-set verifier, so a future S-57 to S-101 bridge can treat both schemes uniformly.

• S57ExchangeSet.VerifyAsync(rootPath, ...) reads CATALOG.031 and validates every entry's CRC against the file on disk, returning per-file results rather than throwing.
• CRC-32 (IEEE/zlib, polynomial 0xEDB88320) is fully implemented via an internal S57Crc32 helper (no new package dependency).
• Checksum and S-63 signature outcomes are tracked as independent dimensions on each S57FileVerificationResult.
• S57VerificationOutcome reuses the S-100 member names byte-for-byte (Ok, NotSigned, SignatureInvalid, CertificateUntrusted, CertificateExpired, FileMissing, CertificateNotFound, Error) and appends checksum-specific NoChecksum / ChecksumMismatch.
• S-63 signature verification is a seam only (IS63SignatureVerifier, S63TrustAnchorOptions), defaulting to NotSigned. The full RSA/SA scheme is intentionally deferred to land alongside future S-63 decryption support.

Notes for reviewers

• CRC byte-order is empirically confirmed, not just assumed. Verified against a real NOAA exchange set (cell US5WA70M): both declared CRCS values (FD6C39B8, A8292AA7) matched the computed big-endian uppercase-hex IEEE CRC-32 exactly. A known-answer test also pins the algorithm.
• AllValid treats NoChecksum as non-failing on purpose: CRCs are optional in S-57 and the CATALOG.031 self-reference legitimately carries none. This is a slight, deliberate divergence from S-100's AllValid (which requires Ok for all) and is recorded for any future bridge unification.
• Real FILE subfields use backslash separators (US5WA70M\US5WA70M.000); path resolution normalizes these and falls back from the short name to the long name.
• The verifier stays directory/rootPath-based to match the existing S57ExchangeSet design, rather than introducing the IAssetSource abstraction the S-100 verifier uses.

Tests

10 new xUnit tests using synthetic CATALOG.031 fixtures (no real ENC data committed), including a CRC-32 known-answer test and coverage of Ok / ChecksumMismatch / NoChecksum / FileMissing, the aggregate predicates, and the signature seam. Full suite: 437/437 passing; clean net8.0 + net10.0 build. README updated.

Closes #6