docs: Rust-core + Python-analysis split proposal

[ingmarschuster]

Summary

Adds docs/rust-core-migration-plan.md — an RFC proposing forge be split into a single cross-platform Rust governance-core binary plus an optional Python analysis pack (forge-analysis), so non-Python repos can adopt forge's governance layer without a Python interpreter.

Changes

• New docs/rust-core-migration-plan.md: problem, target architecture, component split, distribution changes, phased/reversible migration, risks, and a decision gate.
• Registers the doc in REPO_STRUCTURE.md (keeps the repo-structure check green).

Key points

• The split follows an existing seam: the dispatcher already shells out to steps via exit code + code_health/<step>.log, so checks aren't rewritten — orchestration + generic validators move to the binary; Python-AST analyzers stay Python tools the binary invokes like ruff.
• Distribution shifts from git tag to a cross-compile/sign/publish pipeline; adds npm-with-per-platform-binaries (the no-Python win for Node repos); introduces a versioned binary↔pack IPC contract.
• Framed as an RFC with a decision gate: worth doing only for a growing set of non-Python consumer repos.

Breaking Changes

None — documentation only, proposal not yet accepted.