refactor(aztec-nr): keep handshake secrets internal to TagSecretSource

[nchamo]

Summary

• TagSecretSource::obtain_secrets returned AppSiloedHandshakeSecrets for every source kind. That was wrong: the pair only makes sense for handshake-backed sources, and the unconstrained kind had to fake sender_only: 0. It appears this leaked in automatically during a Claude conflict resolution.
• The cross-source interface is now a bare tag secret. resolve_tag_secret returns the Field the tag derives from, and constrain_tag_secret anchors the sequence and emits the sequence nullifier itself. AppSiloedHandshakeSecrets stays where it is honest: the registry wire type, the constrained-delivery helpers, and the source's internal payload.
• This way feels better: tag derivation only ever sees the single concept it consumes.

[AztecBot]

Flakey Tests

🤖 says: This CI run detected 1 tests that failed, but were tolerated due to a .test_patterns.yml entry.

\033FLAKED\033 (8;;http://ci.aztec-labs.com/9198a126c5100b44�9198a126c5100b448;;�):  yarn-project/end-to-end/scripts/run_test.sh simple src/single-node/proving/proof_fails.parallel.test.ts "does not allow submitting proof after epoch end" (117s) (code: 0) group:e2e-p2p-epoch-flakes