types: represent DTLS wire-code identifiers as compact newtypes

[zRedShift]

Follow-up to #134. This is a breaking API cleanup.

This converts DTLS/TLS wire-code identifier types from enums with Unknown(...) variants into compact transparent integer newtypes. Known values remain exposed as associated constants, with the previous enum-style casing preserved to keep call-site churn down.

Scope:

• public/provider-facing wire-code types such as HashAlgorithm, SignatureAlgorithm, CompressionMethod, ContentType, NamedGroup, SignatureScheme, Dtls13CipherSuite, and ProtocolVersion
• DTLS 1.2/1.3 internal message, extension, cipher-suite, curve, and certificate-type code types

The intent is to keep unknown wire values representable while avoiding enum padding/discriminants in parser and error payload paths. This does not intentionally change parser policy, replay behavior, queue behavior, or handshake semantics.

Validation:

• git diff --check upstream/main..HEAD
• cargo fmt --check
• check-snowflake-local.pl upstream/main
• cargo test --all-targets --features rcgen
• cargo clippy --all-targets --features rcgen -- -D warnings
• cargo test --no-default-features --features rust-crypto
• cargo clippy --no-default-features --features rust-crypto -- -D warnings
• cargo test --doc --features rcgen