component-test CLI: compose via wac-graph, not wac script + binary

[lann]

The component-test CLI (already in scope) should perform composition programmatically via the wac-graph crate rather than shelling out to wac with a checked-in .wac script.

Rationale (from the prototype/ work):

• The L2 topology is a fixed shape: instantiate provider → wire suite's test-context import → re-export tests, test-context, factory → (optionally) plug bundle into a runner core. A programmatic graph is simpler and less error-prone than templating a DSL.
• wac plug of provider into suite "succeeds" but silently drops the provider's factory/test-context exports; the resulting bundle then fails runner linking with the opaque type mismatch in instance export 'context'. The tool can make the correct topology the only expressible one and turn residual failures into domain errors (e.g. "suite bundle missing test-context").
• Bundle-then-plug remains the conceptual model (bundles are distributable artifacts); the tool should still be able to emit the intermediate bundle for interop with plain wac-based workflows.

Reference: prototype/bundle.wac + prototype/README.md findings.

[lann]

Additional composition duty from the multi-instance scenario work (#21): the suite-world extension pattern — concrete suites may import domain sidebands beyond test-context (e.g. a signaling mailbox for two-peer tests), and the bundle step composes in the corresponding provider component, structurally identical to the context provider. wac-graph should treat 'runner-provided imports' as an open set: context provider + N domain-sideband providers, all shared-instance, all re-exported by the bundle for the runner core's imports.

[lann]

Done in #86: component-test compose-runner (and run) build the composition programmatically with wac-graph 0.10. The fixed L2 topology is the only expressible one — one shared provider instance feeds the suite's test-context and the runner core's factory/test-context — and the failure modes this issue catalogued are domain errors now: non-suites rejected by name, already-composed inputs redirected to run, and partial bundles (context without factory or vice versa) rejected with a pointer at bundle.wac instead of the opaque type mismatch in instance export 'context'. Bundles stay first-class inputs (tests + test-context + factory re-exports plug straight into the runner core), so bundle-then-plug remains the conceptual model and plain-wac workflows keep working; examples/compose documents both routes.

One sub-item did not land: emitting the intermediate bundle (suite + provider, no runner core) for interop with plain-wac pipelines — compose-runner only emits the fully-composed component. Closing anyway: the core ask (library composition, correct-topology-only, domain errors) is done, and the residual is a small --bundle-only flag someone can hang off compose-runner when a consumer actually wants the distributable bundle artifact — reopen or file under #13's charter then.