docs: fix README Python status (#13 closed; not planned/shipped)

[rmems]

Summary

README still says Python bindings are planned via #13. That issue is closed — update status so docs match reality.

Plan

1. Fix Cross-language notes table
2. Remove or rewrite "when #13 lands" language
3. Keep Julia sister project note accurate

Relationships

• Parent: Epic: plasticity-lab v0.2 — Hardening & honesty #43

Acceptance criteria (shared)

• cargo fmt --check && cargo clippy --all-targets --all-features -- -D warnings && cargo test --all-features green

[linear-code]

LIM-1003