v0.6.0 — full 20-endpoint signed absorption

Absorbs every wrapped-JSON /...-signed endpoint chain shipped on
2026-05-03 in one drop (per all-signed-endpoints-shipped.md):

- 18 single-sig SignedXxxAsync methods on VelocityClient
- 2 §18 committee-quorum SignedCommitteeQuorumForce*Async methods
- SignedActionEnvelope.BuildQuorum + CommitteeQuorumSignature
- SignedEndpointRegistry (20-entry kind→path map)
- Response DTOs (SignedActionWriteResponse, CommitteeQuorumSignedResponse)
- New domain tags + 9 borsh payload structs (Mint/Burn/RegSession/
  RevokeAcct/GrantCap/Grant+RevokeCrossNs/4×§18/FlashSwap)
- Tests for envelope, registry, response DTOs, and every borsh layout

The two legacy binary-envelope endpoints (register-signed,
add-key-signed) keep their original shape; SDK still routes to them
via the existing identity flow.