feat(runtime): Runtime preset bundling components and add-ons

[mfw78]

Summary

Closes #93.

Adds a Runtime preset trait to nexum-runtime that bundles a component set and an add-on list behind a single type, plus a RuntimeBuilder::runtime::<R>() shortcut that consumes a preset directly, so an embedder with no domain extensions can go from configuration to a running node in one call.

This is part of the epic #79 train and merges in stack order.

Changes

crates/nexum-runtime/src/preset.rs is new and defines the Runtime trait: an associated Types lattice, three associated component-builder types bounded by ComponentBuilder<Output = Types::Chain/Store/Ext>, and components()/add_ons() associated functions returning a ComponentsBuilder and an AddOns list. The same file defines CoreRuntime, the domain-free default preset (Chain = ProviderPool, Store = LocalStore, Ext = ()), which bundles PrometheusAddOn as its default add-on.

crates/nexum-runtime/src/builder.rs gains RuntimeBuilder::runtime::<R>(), a new terminal stage (PresetBuilder) reached directly from the initial builder. It exposes with_extensions, with_module_source, and launch. launch builds the preset's Components, keeps the preset's owned Vec<Box<dyn RuntimeAddOns>> alive alongside a borrowed &dyn RuntimeAddOns view across the launch await, and hands both into the existing AssembledRuntime/LaunchRuntime::launch path unchanged.

crates/nexum-runtime/src/addons.rs adds pub type AddOns = Vec<Box<dyn RuntimeAddOns>>, the return type the issue names for add_ons().

crates/nexum-runtime/src/lib.rs registers the new preset module.

crates/nexum-runtime/examples/embed.rs is rewritten to the one-line RuntimeBuilder::new(&cfg).runtime::<CoreRuntime>().launch().await?.wait().await. The prior example's components.logs read-back demo is kept as a module-doc note rather than dropped, since that path only makes sense against the explicit component builder, not a preset.

Test plan

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-features (all suites pass, including the wasm-backed cow_boot supervisor end-to-end fixture, built rather than skipped)
• embed.rs example compiles and matches the one-liner in this description

Notes for reviewer

components() and add_ons() are associated functions with no self, since presets are zero-sized markers; runtime::<CoreRuntime>() needs no value to construct. The three associated builder types (rather than a single opaque return) exist because ComponentsBuilder<C, S, E> is generic over the concrete builder types, and the Output = Types::Chain/Store/Ext bounds are exactly what ComponentsBuilder::build needs, so no extra where-clauses leak to callers of the trait.

The default preset is named CoreRuntime, not the issue text's stale EthRuntime, to match the tree's existing domain-free CoreTypes naming and stay honest that this preset carries no chain-specific behaviour.

Two items from the issue are deliberately left out of scope. The CLI (nexum-cli) is not converted to a preset here: it needs the cow Extension linker hook, which the Runtime trait (components plus add-ons only) does not carry, and wiring that in is a separate decision about whether presets should also provide extensions. Shipping a cow-specific ReferenceRuntime preset in shepherd-cow-host is the natural follow-up once that question is settled; the issue only suggested it as a possible example, not a requirement.

AI Assistance: Claude Code (Opus 4.8 implementation and adversarial review, Sonnet 5 PR authoring) used for the full change.