feat: v2 Phase 1 — foundation, proto schemas, RPC framework, SDK skeleton

New workspace structure with 9 crates. Adds:

- proto/qpq/v1/*.proto: 11 protobuf schemas covering all 33 RPC methods
- quicproquo-proto: dual codegen (capnp legacy + prost v2)
- quicproquo-rpc: QUIC RPC framework (framing, server, client, middleware)
- quicproquo-sdk: client SDK (QpqClient, events, conversation store)
- quicproquo-server/domain/: protocol-agnostic domain types and services
- justfile: build commands

Wire format: [method_id:u16][req_id:u32][len:u32][protobuf] per QUIC stream.
All 151 existing tests pass. Backward compatible with v1 capnp code.

crates/quicproquo-proto/Cargo.toml

@@ -1,22 +1,28 @@
 [package]
-name        = "quicproquo-proto"
-version     = "0.1.0"
-edition     = "2021"
-description = "Cap'n Proto schemas, generated types, and serialisation helpers for quicproquo. No crypto, no I/O."
-license     = "MIT"
+name = "quicproquo-proto"
+version = "0.2.0"
+edition = "2021"
+description = "Protocol types for quicproquo — v1 Cap'n Proto (legacy) + v2 Protobuf (prost)"
 
-# build.rs invokes capnpc to generate Rust source from .capnp schemas.
-build       = "build.rs"
+build = "build.rs"
 
 [dependencies]
+# v1 legacy (Cap'n Proto) — used by existing server/client until rewrite
 capnp       = { workspace = true }
 
+# v2 (Protobuf via prost) — new RPC types
+prost       = { workspace = true }
+prost-types = { workspace = true }
+bytes       = { workspace = true }
+
+[build-dependencies]
+capnpc      = { workspace = true }
+prost-build = { workspace = true }
+protobuf-src = "2"
+
 [lints.rust]
 unsafe_code = "warn"
 
 [lints.clippy]
 # Generated Cap'n Proto code uses patterns that trigger clippy lints.
 unwrap_used = "allow"
-
-[build-dependencies]
-capnpc      = { workspace = true }