Require host dependency staging for fixture matrices

[chubes4]

Summary

• make registry dependency discovery and immutable host package resolution unconditional for executed fixture-matrix batches
• remove the hostDependencyOrchestration run option and SSI_FIXTURE_MATRIX_HOST_DEPENDENCY_ORCHESTRATION environment variable
• update matrix test runtimes to represent the mandatory planning phase

Closes #847.

Verification

• node --test tests/host-dependency-resolver.test.mjs
• npm run test:fixture-matrix (259 passing)
• fixture 87-travel-tours resolves Jetpack 16.0.1 on the host, verifies SHA-256 4bc0a29ebc44da0172093328001bcf0fea622e26754aa71a067dc82b3236417d, stages it as an extra_plugin, and activates it before workloads
• the former 579,884 ms in-Playground download crash is eliminated; the now-visible Jetpack 16 Forms readiness incompatibility is separately tracked in Restore Jetpack 16 Forms provider readiness in fixture runtime #848

AI assistance

OpenAI GPT-5.6 Sol using OpenCode traced the fixture 87 runtime evidence, implemented the mandatory host-staging path, updated tests, and ran verification. Chris Huber directed the architecture and is responsible for every line.