feat: generate the RPC shape for a protocol (surface 4.2)

[Kinflou]

code mode now emits, per FrozenUnit::Protocol, everything comline-runtime's surface-4 contract needs — turning the hand-written stand-ins in the runtime's dispatch_roundtrip / client_roundtrip tests into real generator output.

What's emitted

piece	shape
params	pub struct <Proto><Fn>Params per function that takes args (owned fields)
per-fn error	pub enum <Proto><Fn>Error from the function's throws ordinals — resolved against the FrozenUnit::Error units' ordinal (local or a re-exported import, core#47); empty enum when it throws nothing
union	pub enum <Proto>Error ∪ every ! in the protocol + From<<Fn>Error> impls
provider	pub trait <Proto> — sync, &self, Result<R, <Fn>Error> (schema errors only) + pub const <PROTO>_CALLS: &[&str]
dispatcher	<Proto>Dispatcher<S> impl comline_runtime::contract::Dispatch — index jump table → decode params → run handler → frame the ok/err Envelope
client	<Proto>Client<T, W> wrapping comline_runtime::client::Client — one method per fn → Result<R, CallError<<Fn>Error>>

FrozenUnit::Error gets struct codegen too now (it was skipped entirely before). lib mode's generated Cargo.toml git-deps comline-runtime when any schema has a protocol.

Deferred (noted in code)

• Borrowed generated types (<'de>) — params are owned (String / Vec<T>) for now.
• True one-way — _return: None and KindValue::Unit both render -> () (request/response with an empty ack); fire-and-forget is an unbuilt runtime feature.
• Function.parameters (per-call settings) — not consumed yet.

Tests

tests/compiles.rs — generates a lib crate for a protocol (throwing call, list return, zero-arg call, unit return) and actually cargo builds it against comline-runtime (string assertions can't prove it compiles). Plus expanded string-match coverage in tests/generate.rs. cargo test --workspace green (1 + 5 + doctests).

Follow-up

The generation conformance corpus's protocol golden will need a re-bless once this lands (separate PR — bump its comline-codegen-rust rev, CONFORMANCE_BLESS=1).