Add the audit chain verifier

Implemented proto::ChainVerifier as a pure line-holding state machine:
each line is judged only once the next has arrived, so a Recovery record
can mark the line before it not-a-record. Adds ChainFailure, TornTail,
ChainReport, Location and ChainVerifier, wired through lib.rs and the
matching re-export in audit.rs. 13 chain tests pass; make gate prints
gate: ok.

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

crates/proto/src/lib.rs

@@ -1,6 +1,7 @@
 //! Shared data types and the frame codec for Boxmaker. No policy and no I/O beyond frames.
 
 pub mod audit;
+pub mod chain;
 pub mod class;
 pub mod frame;
 pub mod grant;
@@ -10,6 +11,7 @@ pub mod log;
 pub mod wire;
 
 pub use audit::{ApprovalAnswer, AuditEvent, AuditRecord, DecisionRecord, ResultStatus};
+pub use chain::{ChainFailure, ChainReport, ChainVerifier, Location, TornTail};
 pub use class::DataClass;
 pub use frame::{FrameError, MAX_FRAME, read_frame, write_frame};
 pub use grant::{Constraints, Grant, Mode};