feat: Sprint 3 — C FFI bindings, WASM compilation, Python example, SDK docs

- Create quicproquo-ffi crate with 7 extern "C" functions: connect,
  login, send, receive, disconnect, last_error, free_string
  (produces libquicproquo_ffi.so and .a)
- Feature-gate quicproquo-core for WASM: identity, hybrid_kem,
  safety_numbers, sealed_sender, app_message, padding, transcript
  all compile to wasm32-unknown-unknown
- Add Python ctypes example (examples/python/qpq_client.py) with
  QpqClient wrapper class and CLI
- Add SDK documentation: FFI reference, WASM guide, qpq-gen generators
- Update Dockerfile for quicproquo-ffi workspace member

crates/quicproquo-ffi/Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "quicproquo-ffi"
+version = "0.1.0"
+edition = "2021"
+description = "C FFI bindings for quicproquo messaging operations."
+license = "MIT"
+
+[lib]
+crate-type = ["cdylib", "staticlib"]
+
+[dependencies]
+quicproquo-client = { path = "../quicproquo-client" }
+tokio = { workspace = true }
+anyhow = { workspace = true }
+serde_json = { workspace = true }
+hex = { workspace = true }
+
+[lints]
+workspace = true