Adds the crypto primitives the 1.30.0 per-session broker presence flow
needs: canonicalSessionAttestation/canonicalSessionHello bytes, and
verifySessionAttestation/verifySessionHelloSignature with TTL bounds
(≤24h) plus standard ed25519 + skew checks.
10 unit tests cover the hostile cases — expired attestation, over-TTL,
wrong-key signing, tampered fields, and the "attacker captured the
attestation but doesn't hold the session secret key" scenario.
No wire changes yet — types and dispatch land in the next two commits.
Spec: .artifacts/specs/2026-05-04-per-session-presence.md.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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