docs: reconcile SpecSafe assurance boundary

[LaloLalo1999]

Summary

• define SpecSafe as repository-local assurance
• reserve the normalized cross-project ledger for Telar
• document the evidence journal, future gates, adapters, security and deletion tests

Validation

• typecheck passed
• 111/111 tests passed
• clean branch was constructed from current origin/main, so no foreign local commits are included

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