fix(bus): require authenticated delivery provenance

[hartsock]

What this PR does

• Makes Inbound carry typed transport-authenticated peer provenance and rejects Unbound delivery by default.
• Centralizes envelope verification, carrier/signer, same-user, and direct-recipient admission ahead of replay-state mutation, including validation of the local certificate and root identity.
• Binds Iroh Hello certificates to the QUIC session and intended target in both directions, and gives InMemoryTransport a fixed registered identity.

Closes #79

Test plan

• CARGO_TARGET_DIR=/private/tmp/agent-mesh-target-79 just check
• CARGO_TARGET_DIR=/private/tmp/agent-mesh-target-79-cov just cov-ci (75% floor)
• Added hostile admission tests for unbound delivery, signer/root mismatch, stranger roots, bad signatures, CID mismatch, wrong recipients, and deterministic real-QUIC relayed-envelope rejection with a same-envelope positive control.

Out of scope

• Authorized relay semantics; this change admits only a direct carrier whose authenticated identity matches the envelope signer.
• SSH session proof-of-possession and production SSH ingress/trust policy.
• Reply-correlation responder binding, handled by the stacked security(bus): bind reply correlations to the expected responder principal #80 change.

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