net: re-countersigning supersedes a party's own earlier signatures

[samlown]

Observed live after the first complete verification round: the published envelope carried four signatures — the subject's, the verifier's, and two Authority countersignatures (the registration round's and the verification round's), with more to come on every renewal.

Spec §5.3 now requires supersession: a party re-countersigning an envelope MUST replace its own earlier countersignatures (it may never touch another party's). Beyond the clutter, this is a correctness rule — a lingering older signature could assert a verifier the Authority has since revoked, and consumers scanning all signatures would still see it as verified until the old exp passed. Steady state is exactly three signatures: subject, Authority (with verifier), verifier.

Companions implementing it: invopop/gobl.lookup and invopop/gobl.kyb.sandbox strip their own earlier signatures before countersigning.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.99%. Comparing base (5b286a7) to head (7dad329).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #926   +/-   ##
=======================================
  Coverage   94.99%   94.99%           
=======================================
  Files         327      327           
  Lines       17732    17732           
=======================================
  Hits        16845    16845           
  Misses        547      547           
  Partials      340      340           

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