net: streamline spec and require independent verifier

[samlown]

• Rewrite the GOBL Net README as a concise RFC-style specification.
• Clarify exported package and method documentation.
• Resolve contradictions around JWKS discovery, signature ordering, and verification APIs.
• Require registration authorities and identity verifiers to be distinct participants; self-referential verifier claims now produce registration-only endorsements.
• Condense and reorganize the unreleased changelog entries.

Pre-Review Checklist

• Opened this PR as a draft
• Read the CONTRIBUTING.md guide.
• Performed a self-review of my code.
• Updated verification tests; go test ./net passes.
• Examples are not applicable.
• Source links are not applicable.
• Generated schemas and regime data are unaffected.
• git diff --check passes.
• Reviewed nil handling.
• Updated the CHANGELOG.md with an overview of my changes.
• Marked this PR as ready for review.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.99%. Comparing base (c7be72d) to head (cd91fcc).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #928   +/-   ##
=======================================
  Coverage   94.99%   94.99%           
=======================================
  Files         327      327           
  Lines       17732    17732           
=======================================
  Hits        16845    16845           
  Misses        547      547           
  Partials      340      340           

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