feat(client): add peer-pinned TinyDocs host calls - #47

Adds unit tests for the client's error classification (invalid-input vs. unavailable vs. failed), the redaction of peer-supplied text in Debug/Display, and the empty-string SHA-256 digest; each assertion matches the classify_failure/classify_sdk_error definitions and the DocumentCallError formatting I was shown, and the digest literal is the correct SHA-256 of the empty vector. No defects found; the only thing I could not verify first-hand is the exact ModuleCallFailure Display text asserted in raw_message, since the vendored tinybus submodule is not checked out. 2 files could not be reviewed: crates/tinydocs-client/src/client.rs, crates/tinydocs-client/tests/recording_broker.rs.

No findings.