refactor(codegen): distribute generator through moonx

[Milky2018]

Summary

• split typed command generation out of proton_cli into the independently published moonbit-community/proton_codegen module
• ship proton_codegen as a WASM executable and invoke it through a pinned moonx coordinate from generated Moon prebuild rules
• remove CLI-owned codegen commands and update repository generation, release, CI, and scaffold smoke coverage

Why

Code generation is a build dependency of MoonBit packages, not an implicit responsibility of proton_cli build, dev, or package. Making it an explicit dev_build rule gives Moon the real input/output graph, regenerates output for every normal Moon build path, and avoids directory scanning or CLI lifecycle coupling.

Validation

• moon fmt
• moon info
• moon -C codegen check --target wasm --deny-warn
• moon -C codegen test lib --target wasm --diagnostic-limit 80
• moon -C cli test new --target native --diagnostic-limit 80
• node scripts/verify_generated.mjs --skip-prebuilt-abi
• moon check --target native --deny-warn
• PROTON_NATIVE_DIST=$PWD/native/dist node scripts/e2e_scaffold_source_smoke.mjs
• moon -C codegen publish --dry-run completed package creation and extracted-package validation; registry upload was denied because the local account is not the moonbit-community publisher