feat: freeze v1 transport contracts (#4)

Closes #4

docs/contracts/README.md

@@ -0,0 +1,18 @@
+# Versioned contracts
+
+Tracking: #4
+
+The v1 contract is defined by three artifacts that are reviewed and versioned
+together:
+
+- [HTTP v1 semantics](http-v1.md)
+- [UDP v1 wire format](udp-v1.md)
+- [Generated OpenAPI 3.1 document](../api/rendezvous-v1.json)
+
+The public .NET types live in `FinalFactory.Rendezvous.Contracts`, target
+`netstandard2.1`, and contain no Server, Godot, or LiteNetLib dependency. Golden
+vectors and a public-API snapshot make accidental wire or source compatibility
+changes fail the normal test gate.
+
+Any incompatible change requires a new contract version. Additive JSON fields
+may be introduced within v1 because v1 readers ignore unknown object members.