Issue Title: Proposal: "Carbon Core" – A Stable, Standalone Profile without C++ Interop for Zero-Jitter Engines

Hello Carbon Team,

I am currently architecting a high-performance, deterministic engine (four-stage reactor logic involving real-time astrophysical data and quantum state calculations). Carbon’s modern design concepts are exactly what the future of systems programming needs. However, the heavy focus on C++ interoperability and the resulting long roadmap is a massive bottleneck for greenfield projects that do not carry legacy code.

I am requesting the immediate prioritization of a "Carbon Core" compiler profile. This would be a stable, stripped-down version of Carbon that completely drops C++ backward compatibility and focuses strictly on a standalone, low-level toolchain.

We need a stable release of these core features to build production-ready, zero-jitter systems now:

1.) 100% Standalone Compilation: Zero C/C++ interop overhead. A pure, clean Carbon compiler.

2.) Aggressive Comptime Execution: Full support for offloading complex vector transformations and heavy state calculations entirely to compile-time.

3.) Zero-Jitter & Strict Determinism: Flawless, cycle-accurate memory control without latency spikes.

4.) Native Choice Types & Pattern Matching: For bulletproof state-machine implementations and heuristic logic protocols.

5.) Definition-Checked Generics: Guaranteed type safety at the definition level to eliminate runtime ambiguity.

Given the massive capabilities of current AI coding agents (including Google's own internal infrastructure), accelerating a clean, standalone parser and LLVM backend for only these core Carbon mechanics should be highly feasible. There is a significant demand for Carbon as a pure, modern systems language today. We want to build on your architecture, but we need a stable foundation for greenfield projects without the C++ baggage.

Best regards, Micha