PROP-220: verify chain-bound Repl creation timestamps

[pellicceama]

What changed

The Go Repl identity verifier now exposes a Repl creation time only when a certificate claim binds the value. It rejects a body that differs from the claim and ignores an unbound body value, so workload code cannot backdate a Repl with REPL_IDENTITY_KEY.

This PR mirrors the contract from https://github.com/replit/goval/pull/22816. It also preserves the timestamp when a caller signs an audience-specific child identity.

References PROP-220

Test plan

• go test ./...

Rollout

Release this module after the Goval contract merges. Goval must update its Vault renewal consumer to this release before web starts sending the timestamp.

This change is additive and safe to revert before timestamp emission starts.

~ written by Zerg 👾 (volatile-tempest-e716)

[codecov]

Codecov Report

❌ Patch coverage is 78.72340% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.42%. Comparing base (2b01855) to head (9ea4721).

Files with missing lines	Patch %	Lines
verify.go	56.52%	6 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #50      +/-   ##
==========================================
+ Coverage   51.21%   52.42%   +1.20%     
==========================================
  Files           7        7              
  Lines         697      723      +26     
==========================================
+ Hits          357      379      +22     
- Misses        291      293       +2     
- Partials       49       51       +2     

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