refactor(test): add BootScenario and Refusal boot helpers

[mfw78]

What

Adds BootScenario, Entry, Booted and Refusal in crates/nexum-runtime/src/test_utils/scenario.rs, a new 636-line module that collapses the tempdir, write-manifests, engine-config and boot ritual into one builder over the real Supervisor::boot path.
BootScenario::new() roots a scenario on the CoreRuntime preset with a fresh redb store and an empty provider pool; over() takes caller-supplied Components<T> for other lattices.
module()/adapter() accept anything convertible into Entry, and each entry can name its own wasm component plus, for adapters, HTTP and messaging operator grants.
boot() builds the linker and calls Supervisor::boot directly; expect_refusal() panics if boot succeeds and otherwise wraps the error in Refusal, which centralises names/lacks context-chain assertions.
ManifestSource is promoted out of harness.rs into manifest.rs so both the harness and the new scenario share one enum, renamed from None/Path/Inline to Beside/Path/Toml to describe what each variant does rather than its shape, plus a resolve method and From impls for TestManifest, String and PathBuf.
test_utils/mod.rs re-exports the new scenario module and its public types.

Why

Part of #83.

T1 introduced the shared manifest builder; T2 gives the module tree work an equivalent for booting, so later slices in #83 can express a multi-component or granted-adapter boot, or a discovery/path/section refusal, without hand-rolling the tempdir and config wiring each time.
This PR retargets to main once the chain-backend PR ahead of it in the stack merges.

Testing

New tests in test_utils/scenario.rs and test_utils/manifest.rs cover a single-component scenario boot, dispatch and alive-count; two entries with distinct per-entry components booting side by side; a WASI clock override reaching the booted guest via a pinned wall-clock readback; an unknown module capability and an unregistered adapter kind each refusing before compile; a component with no manifest anywhere refusing on the discovery path; a nonexistent explicit manifest path refusing by name; a wired extension claiming a manifest section that an unwired one refuses; expect_refusal panicking when boot succeeds; the scenario store living under the scenario directory; entries carrying their component, manifest and operator grants through split(); and the ManifestSource resolve/From conversions round-tripping for Beside, an explicit path and inline TOML.

AI Assistance

Implement claude-fable-5, red-team claude-opus-5, PR claude-sonnet-5