refactor: modularize VM runtime with V8 defaults

[NathanFlurry]

• Modularize the VM kernel, driver, and executor boundaries while keeping V8 as the default for JavaScript, Python, and WASM.
• Carry the current POSIX, process, event-pump, package, and client behavior into the restructured implementation. Keep Obscura experimental.
• Make Binaryen and Wasmtime opt-in; the default 169-command tool rebuild passes without invoking Binaryen.
• Validate with workspace build/typecheck/Clippy, 417 Core tests, 7 Rust process tests in serial and parallel, Obscura tests, and the WASM import audit.

Draft performance note: the freshly rebuilt command catalog is 18.8% larger than the previously staged artifacts, including an 80.9% DuckDB increase. That comparison also includes compiler and exception-format changes; end-to-end performance has not been re-benchmarked.

[railway-app]

🚅 Environment agentos-pr-2040 in rivet-frontend has no services deployed.

6 services not affected by this PR

• website
• sentry-tunnel
• frontend-cloud-ee
• kitchen-sink
• frontend-cloud
• mcp-hub