Add curve, dns-names, and ip-addrs fields; improve documentation

[sweis]

Summary

This PR adds three new certificate field extraction commands (curve, dns-names, ip-addrs) and significantly improves the README documentation with structured tables for available fields, check types, and verify options.

Key Changes

New Features

• New curve field: Extract EC curve names (e.g., P-256, P-384, P-521) from EC certificates
• New dns-names field: Extract DNS names directly from the SAN extension
• New ip-addrs field: Extract IP addresses (IPv4 and IPv6) directly from the SAN extension
• Added corresponding library methods in xcert-lib/src/fields.rs:
  • CertificateInfo::curve() - returns EC curve name
  • CertificateInfo::dns_names() - filters SAN entries for DNS names
  • CertificateInfo::ip_addresses() - filters SAN entries for IP addresses

Documentation Improvements

• Converted available fields list to a structured markdown table with descriptions
• Added check types table documenting expiry, host, email, and ip checks with their value formats
• Added verify options table documenting all command-line flags with descriptions
• Clarified duration format units (s, m/min, h/hr, d/day, w/week, month, y/year)
• Improved JSON output documentation with clearer explanation of succeeded vs failed counts
• Fixed terminology: renamed passed to succeeded in JSON batch summary to better reflect actual semantics
• Fixed CRL option name in features section: --crl-file → --CRLfile

Implementation Details

• Added Curve, DnsNames, and IpAddrs variants to the FieldName enum
• Implemented field extraction logic with appropriate error messages for non-applicable certificate types
• Added color formatting support for new fields (curve as string, dns-names and ip-addrs as strings)
• Updated help text and examples to showcase new fields
• Maintained backward compatibility with existing field names and aliases

JSON Output Changes

• Renamed passed to succeeded in batch operation summaries to clarify that it counts files processed without errors, not checks that passed
• Added clarification in documentation that for check and verify commands, individual pass/fail status is in per-result data, not in the summary

https://claude.ai/code/session_01MV6PaFHhxHFhjMiaU7zDF8