proxy-config: make domain matching deterministic (case-insensitive + IDNA A-label canonicalization)

[fazoncore]

The draft specifies wildcard semantics and trailing-dot equivalence. One remaining interop ambiguity is domain canonicalization: DNS names are case-insensitive and internationalized domains require IDNA processing.

Impact:
Different implementations may match domains differently (Example.COM vs example.com; U-label vs A-label), leading to inconsistent application of destination rules.

Suggested normative clarification:

• Clients MUST compare domain names case-insensitively.
• Clients MUST perform matching on a canonical IDNA A-label form (convert any U-label input to A-label before matching).
• Keep existing trailing-dot equivalence and “no search suffix” behavior as already specified.

[fazoncore]

Suggested minimal interop clarification (no design change):

• Domain matching MUST be case-insensitive (DNS names are case-insensitive).
• Internationalized domain names MUST be matched on a canonical IDNA A-label form (convert any U-label input to A-label before comparison).

This keeps existing wildcard/trailing-dot/no-search-suffix behavior, but removes cross-implementation ambiguity for domain matching.