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

• Depends on xs #144 (with_base_engine). cross-stream temporarily tracks the xs feat/engine-base branch; revert to a release once #144 lands.
• No Engine::new split: main's is already clean (the server job lives on www-next-gen; the split applies there if that is the target).
• Test: test_processor_base_command_surface. Lib suite green (85). test_eval_with_plugin fails only for a missing nu_plugin_test binary (environmental).