RFC: Subscription Typed Interrupt Payload

[johnthecat]

RFC: docs/rfcs/subscription-typed-interrupt-payload.md.

First of a four-PR stack: Subscription Typed Interrupt Payload (#631) → Worker Lifecycle (#632) → Unified Renderer (#633) → Input Modality (#634). Each later RFC builds on the ones before it, so they merge in order.

Why this RFC exists

A live subscription cannot fail. Subscription<Item> yields items only, so when the platform stream behind theme.subscribe errors, the runtime drops the error and the product's theme freezes. Methods declared Result<Subscription<Item>, CallError<E>> send a typed _interrupt only at start, and the generated client reports a plain method's failure as complete. Host-initiated streams have no way to end with an outcome other than a bare decline byte.

The Unified Renderer's render stream and the Input Modality's Input::request both need a stream that ends with a typed value, so this comes first.

What it defines

• Subscription<Item, Interrupt>: a stream of Result<Item, Interrupt> whose first Err is terminal and travels as the _interrupt payload. It replaces both Subscription<Item> and Result<Subscription<Item>, CallError<E>>.
• An empty _interrupt payload is a normal end, delivered as complete. _stop is unchanged. No protocol version bump.
• A method chooses CallError<E> for a failure-only end, or Result<(), CallError<E>> when a normal end carries meaning. Subscription::interrupted(value) replaces Subscription::empty().
• Codegen emits ObservableLike<Item, Reason> and an interrupt decoder for every subscription; a product handler for a host-initiated method returns Subscription<Item, Interrupt> and its typed error crosses the wire intact.

Implementation

Part of this PR. The RFC lands first so the shape can be reviewed on its own.

• truapi: Subscription<Item, Interrupt>, Subscription::interrupted, the 16 subscription signatures and their default bodies
• truapi-codegen: one subscription kind in rustdoc classification, dispatcher and TS emitters, playground metadata; goldens
• truapi-server: subscription_stream maps Err to SubscriptionOutput::Interrupt; platform stream errors forwarded for theme, locale, chat list and preimage; host-initiated caller returns the declared interrupt type
• @parity/truapi: createObservable treats an empty payload as complete, product-side host-initiated handlers encode a typed interrupt
• Tests: wire_result_shape.rs, subscription.rs, client.test.ts

[pgherveou]

lgtm, but please let's stop making RFC PR.
Let's just open an RFC issue that we can comment on and make a PR that implement it.

They should be also easier to parse and trimmed of most Claude bloat

[johnthecat]

Issues: #664