policy: verify every known signature; proof: decode base64 strictly

[benbirt]

TLogPolicy.Verify used to stop at the first log key whose signature verified, and Satisfied opened the note once per witness, stopping as soon as the quorum was met. Signatures from other known keys were never looked at, so a checkpoint carrying an invalid signature from a known log or witness could still be accepted.

c2sp.org/signed-note says that if a signature's key name and ID match a known key, verifiers MUST verify it, and SHOULD reject the whole note if it fails; c2sp.org/tlog-proof likewise says to "verify all cosignatures for witnesses known to the verifier". Verify now opens the checkpoint once with all of the policy's log and witness keys, so note.Open checks every known signature and fails on any invalid one, then applies the log-origin and quorum rules to the verified signatures. Satisfied does the same with the witness keys. Signatures from unknown keys are still ignored.

Separately, c2sp.org/tlog-proof requires decoders to reject non-canonical base64, so TLogProof.Unmarshal now decodes the extra data and inclusion hashes with base64.StdEncoding.Strict(), which rejects non-zero padding bits. Other formats (checkpoints, vkeys, signed notes) are left unchanged, as their specs don't require this.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.03922% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.47%. Comparing base (fa00c16) to head (8b04f1f).
⚠️ Report is 194 commits behind head on main.

Files with missing lines	Patch %	Lines
policy/tlog_policy.go	97.95%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #286      +/-   ##
==========================================
+ Coverage   82.57%   87.47%   +4.90%     
==========================================
  Files           5       10       +5     
  Lines         241     1030     +789     
==========================================
+ Hits          199      901     +702     
- Misses         30      128      +98     
+ Partials       12        1      -11     

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