Add 0.23 roadmap: registry production, Rust tooling port, RGB++/Fiber, off-chain sessions

Drafts CELLSCRIPT_0_23_ROADMAP.md with four pillars: public registry
deployment on cellscript.dev, Python test/fixture scaffolding ported to
Rust, deeper RGB++/Fiber integration, and an Off-Chain Session Runtime
profile aligned with the Myelin Session L2 plan. Wires the 0.23 entry into
both roadmap index files.

Author: a19q3