Add net module (v0.8.0)

[vhrcgcom]

Summary

• Url — validated URL (http/https/ftp/ftps/ws/wss), scheme+host normalised to lowercase
• Domain — domain without scheme, lowercased, at least two labels
• IpV4Address — IPv4, rejects leading zeros in octets
• IpV6Address — IPv6, normalised to canonical compressed lowercase form
• IpAddress — IPv4 or IPv6, tries v4 first
• Port — u16 in range 1–65535
• MacAddress — accepts colon/hyphen/dotted formats, stored as lowercase colon-separated
• MimeType — type/subtype, lowercased, parameters accepted
• HttpStatusCode — u16 in range 100–599, with is_success() etc. helpers
• ApiKey — non-empty; Display masks all but last 4 chars

Extra dep: url (already used by contact).

Closes #46
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Type of change

• New value object / feature

Checklist

• ValueObject trait (new / value / into_inner)
• TryFrom<&str> where applicable
• Display
• serde cfg_attr
• Tests (83 passing)
• cargo fmt + clippy clean
• ROADMAP.md updated
• README.md updated
• docs/net.md added