docs: rewrite README against current project state

[MarcosBrendonDePaula]

Summary

This PR rewrites the project README against the current repository state and removes claims from the deleted engine architecture.

The new README:

• describes the actual compiler/runtime boundaries and current workspace crates;
• documents the commands implemented by rts-cli instead of the removed rts apis command;
• separates verified runtime behavior from roadmap and experimental UI work;
• explains the current source-build requirement for AOT compilation;
• documents the scope and limitations of the Bun/Node/RTS parity corpus;
• links to the canonical engine, Node compatibility, UI, testing, and contribution documents;
• removes broken generated links and the unsupported “single binary” implication.

The README is intentionally shorter and more operational. Detailed architectural rules remain in the crate READMEs and docs/engine/.

Validation

• git diff --check
• bash -n scripts/cross_runtime_check.sh
• Checked every relative README link against the checkout.
• Checked CLI command names against crates/rts-cli/src/cli/mod.rs.
• Checked that the new workspace map uses crates present in Cargo.toml.

A Rust build/test run was not possible in this environment because cargo is not installed. This PR changes documentation only.

Review requested

Please review the factual accuracy of the current-status, AOT, Node compatibility, and cross-runtime sections, with particular attention to the intended public onboarding path.