feat: handshake codegen — IR_HASH + connect / serve helpers

[Kinflou]

The codegen half of runtime#8. A schema with a protocol now also emits:

pub const IR_HASH: u64 = 0x…;   // fingerprint of this file's frozen IR

impl<T: Transport, W: WireFormat> ChatClient<T, W> {
    pub fn connect(transport: T, format: W) -> Result<Self, RuntimeError> {
        let hs = Handshake::new(IR_HASH, format.name(), FRAMING_DATAGRAM, 0);
        Ok(Self(Client::connect(transport, format, hs)?))
    }
}

impl<S: Chat> ChatDispatcher<S> {
    pub fn serve<T: Transport, W: WireFormat>(self, transport: &mut T, format: W)
    -> Result<(), RuntimeError> {
        let hs = Handshake::new(IR_HASH, format.name(), FRAMING_DATAGRAM, 0);
        Server::new(self, format).serve_handshaked(transport, hs)
    }
}

ChatClient::new / a bare Server::serve still work for "misaligned mode".

IR_HASH is FNV-1a over the frozen units' Debug form — deterministic per generator version; a generator bump changes it, which is correct (different codegen ⇒ different handshake identity). A canonical cross-language hash from core's CAS, threaded through GenRequest, is the eventual form (noted in the code). Only emitted when the schema has a protocol.

Bumps the generated crate's comline-runtime pin to the handshake rev.

Tests

tests/generate.rs — IR_HASH + both helpers for a protocol; no IR_HASH / comline_runtime for a protocol-less schema (8 string tests). tests/compiles.rs builds a protocol crate against comline-runtime, so the helpers compile too. Clippy clean.

Follow-up

A cli PR: switch the end-to-end test's driver to ChatClient::connect / ChatDispatcher::serve so the handshake runs through the real pipeline.