feat(truapi): Pocket modality RFC and protocol spec

[valentinfernandez1]

Tracking issue: #563

What this defines

Pocket is a collection of product-backed cards owned by the host. Each card shows a product's key information at a glance and opens the product when tapped.

The host renders each collapsed card natively from a CustomRendererNode tree that the product's worker streams. Tapping a card opens the product's Widget executable in a WebView.

A product cannot add a card on its own. The user adds one by following a Pocket deeplink and approving a host dialog that shows the card as it will look. Both the user and the owning product can remove a card. Humanity, Balance and Scarcity are privileged cards. They are always present and nobody can remove them.

Chat and Pocket share the product's single Worker executable. Its lifetime is a reference count: one reference per active chat, one per visible card. The host terminates the worker when the count reaches zero. When the worker starts, its entry module runs under a bounded time and memory budget.

The RFC also defines the deeplink format and where card definitions are published:

• Deeplinks: polkadot://<product>.<tld>/-/pocket/add?card=<id> offers to add a card, and /-/pocket/open?card=<id> expands one. The first path segment - is reserved for host-handled targets.
• Card definitions: the Worker manifest gains a pocket.cards list. Each entry names the card and points at a static preview tree inside the product archive, so the approval dialog runs no product code.

Protocol surface

Method	Direction	Ids
pocket.list_subscribe	product to host	198 to 201
pocket.remove_card	product to host	202 to 203
pocket.action_subscribe	product to host	204 to 207
pocket.card_render	host to product	208 to 211

The Pocket trait is available to the Worker execution only. Every payload travels in a V1 envelope. A card_render item encodes byte for byte like a chat custom-message render item, so a host that decodes one decodes the other.

The renderer tree types (CustomRendererNode, its props, modifiers and tokens) live in truapi::v01::renderer and are re-exported through truapi::latest. Chat uses them from there. Their wire encoding is unchanged.

Scope

This PR contains the RFC and the crate-level spec only.

truapi-server implements the trait with the default bodies. A host without a Pocket surface answers remove_card with Unavailable, ends the two subscriptions with an empty Interrupt frame, and never opens card_render. ProductRuntimeControl::render_pocket_card is the entry point a host calls to open a face stream.

Pocket behaviour in the runtime, the platform trait, the native bindings and the CLI is tracked by the "Implementation, rust-core" task in #563.