Rewrite peer_id module with opaque API and migrate all call sites

[uink45]

Summary

• rewrite peer_id around an opaque heap-owned peer_id_t and new peer_id_new_* constructors
• remove legacy peer_id wrapper entry points and migrate in-tree users to peer_id_text_write*, peer_id_free, and peer_id_equal
• internalize peer-id protobuf/key-operation internals and remove per-key public headers
• update host/protocol/interop/gossipsub code paths for the new ownership and text encoding APIs
• rewrite peer-id tests and refresh peer-id/identify docs for the new API surface

Validation

• ctest --preset macos-full --output-on-failure
• local pre-push fast-lane smoke checks (triggered by push)