feat: let a Store carry a base engine that processor engines clone

[cablehead]

What

Store::with_base_engine(EngineState) lets an embedder set a base engine the processors clone. prepared_base clones it (or the default Engine::new()), then layers the store commands. No base set: behaviour unchanged.

Why

prepared_base gave embedders nowhere to add commands, so http-nu's .mj never reached services or actors. EngineState is the shared type, so http-nu builds a base and hands it over.

Notes

• Model: ADR 0007 (prepare a base once, clone per subsystem).
• Test: src/nu/test_base_engine.rs. Suite green.
• http-nu bug: src/nu/commands/append_command.rs :: input handling #48 builds on this.
• check.sh clippy flags pre-existing result_large_err in untouched cat_stream_command.rs (stable 1.96 vs MSRV 1.93.1); CI's non-strict clippy is unaffected.