chore(abnf): explicit ABNF clarification

[rmlearney-digicatapult]

Tighten did:webvh identifier and DID-to-HTTPS parsing rules to remove ambiguity and reference DID Core.

• Method-Specific Identifier section now explicitly layers did:webvh constraints on top of DID Core ABNF, with clearer productions for SCID, domain, optional percent-encoded port, and path segments
• Deterministic parsing requirements for percent-encoded port handling, including explicit disambiguation of terminal percent-encoded colon + digits and constraints on where percent-encoded colons may appear
• Explicit normalization and validation requirements for domain and path processing (single decode pass, IDNA handling, IP-address rejection, path safety checks, canonical encoding expectations)
• Clarified that DID URL path/query/fragment components are outside method-specific-id parsing and must not be interpreted as SCID/domain/port/path segments

These changes are consistent with did:web as well (except for the existence of SCID), just spelled out more explicitly.