feat(runtime): mock backends and a MockRuntime assembly via the builder

[mfw78]

Summary

Closes #94; part of the epic #80 train, merges in stack order.

Ships engine-side test fakes behind a test-utils feature so a runtime can be assembled and launched entirely through the public builder, on mocks, with no CLI, disk, or network.

Changes

crates/nexum-runtime/Cargo.toml adds [features] test-utils = [] plus a self dev-dependency (nexum-runtime with features = ["test-utils"]) so the crate's own tests and doctests see the mocks without every invocation passing --all-features.

src/test_utils/chain.rs adds MockChainProvider, a Clone + Send + Sync ChainProvider with programmable on_method/on_request responses keyed by (method, params), recorded_requests, and push_block/push_chain_log feeding futures::channel::mpsc receivers returned as the boxed BlockStream/ChainLogStream. Unprogrammed methods return ProviderError::UnknownChain, matching ProviderPool::empty(). A second subscribe_* after the receiver is taken parks on stream::pending() so a reconnect loop cannot busy-spin.

src/test_utils/store.rs adds an in-memory MockStateStore/MockStateHandle (Arc<Mutex<HashMap>>, no redb or disk) that mirrors the redb-backed store's behaviour: the same InvalidNamespace message on an empty namespace, Ok(None) on an absent key, idempotent delete, and prefix-filtered, sorted list_keys.

src/test_utils/builders.rs adds a generic pass-through Prebuilt<T> ComponentBuilder.

src/test_utils/types.rs adds the domain-free MockTypes lattice (Chain = MockChainProvider, Store = MockStateStore, Ext = ()).

src/test_utils/mod.rs re-exports the above, adds mock_components/mock_components_from helpers, a module doctest, and the M0 acceptance and mock unit tests.

src/supervisor.rs deletes the test-only DefaultSupervisor::empty_for_test constructor and corrects the one doc line that named it.

src/supervisor/tests.rs moves both former empty_for_test call sites onto a zero-module Supervisor<MockTypes> booted through the real Supervisor::boot path via a new boot_mock_supervisor helper; empty_supervisor_returns_no_subscriptions becomes #[tokio::test].

Cargo.lock gains the one line for the self-dep edge, required for CI --locked.

Test plan

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-features --no-fail-fast (0 failed; new test_utils and rewritten supervisor::tests cases ran and passed, not skipped)
• cargo test -p nexum-runtime --locked (no --all-features, confirms the self dev-dependency turns the feature on for just test)
• cargo doc --workspace --no-deps --all-features with RUSTDOCFLAGS=-D warnings

All runs were under nix develop with an isolated CARGO_TARGET_DIR. Module wasms (example, M2/M3, fixture bombs) were built in-tree before the workspace test run.

Notes for reviewer

empty_for_test was deleted outright rather than replaced by a new bespoke test constructor. The two supervisor tests now boot through the real, public Supervisor::boot path with mock_components() and a default module-less config, which exercises real assembly instead of a synthetic shortcut and better matches the epic's "launched entirely through the public builder" intent.

The M0 acceptance test drives the full RuntimeBuilder::new(cfg).with_types::<MockTypes>().with_components(...).with_add_ons(&[]).launch() chain, asserts it reaches supervisor boot and bails cleanly on no modules, then does a direct request round-trip against the mock chain to prove it serves and records. A full fake-driven block-dispatch end-to-end test was deliberately left out of this hermetic acceptance test; it needs a wasm guest and a written manifest (disk), and the channel plumbing it would exercise is already covered by subscribe_blocks_yields_pushed_headers.

futures::channel::mpsc was chosen over tokio for the mock streams: it is already a dependency, avoids an unused-crate-dep risk, and keeps the wasm cone clean.

Response keying uses (&'static str method name, params) rather than adding Hash to the public wire-surface ChainMethod enum, to keep that closed enum's derive set untouched.

Follow-ups intentionally left out of this PR: scriptable error and stream-end injection on the mock chain (so reconnect and backoff can be unit-tested on fakes); a full fake-driven block-dispatch end-to-end test gated on the wasm fixture; promoting a MockRuntime preset once presets can accept pre-built instances rather than only building from config; a docs page for the test-utils surface (the module doctest carries the minimal example for now).

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