feat: processor engines get http-nu's context-free commands (.mj etc.)

[cablehead]

What

Hand the xs processors (actor, service, action) a base engine with http-nu's .mj, .md, and highlight commands, so they work in a service or actor, not just an HTTP handler.

• Engine::add_processor_commands(&store): store-backed .mj/.mj compile, .mj render, .md, .highlight*.
• Store::init sets this base before the processors spawn; prepared_base clones it per spawn.

Why

.mj was registered only on the HTTP engine, so using it in a service failed. Model: xs ADR 0007.

Notes

• Uses Store::with_base_engine, released in cross-stream 0.14.0, which main now depends on. Rebased onto main; the temporary xs branch pin and the 0.13.4 repoint are gone, so this PR no longer touches Cargo.toml or Cargo.lock.
• No Engine::new split: main's is already clean.
• Test: test_processor_base_command_surface.
• Verified end to end with http-nu eval --store --services: an actor calling .md emits md.out with the rendered HTML. On main the same actor fails with "Command .md not found".