chore: update Yarn and expose Calcit source

[tiye]

Summary

• Pin Yarn 4.18.0 in the package, CI, and contributor instructions while retaining the node-modules linker.
• Ignore Yarn cache and Plug'n'Play artifacts; keep generated output out of git.
• Stop marking calcit.cirru as generated, so Calcit source remains reviewable.
• Keep Calcit 0.18.1 and existing module versions unchanged.

Validation

• yarn install --immutable (Yarn 4.18.0; scripts disabled).
• caps --strict --ci && caps verify --toolchain.
• Four unit tests, client/server check-only, dynamic-method and quality gates passed.
• Client/server JS generation, client-generation smoke test, and Vite production build passed locally.