refactor: make Keel canonical for code generation

[turinglambdaai]

Summary

• use Keel app-schema objects as the canonical in-memory input to Swift, C++, and Kotlin generation
• keep one explicit, fail-closed Keel-to-RVT1 type adapter while preserving generated sources and schema snapshots
• share the Keel module instance across isolated codegen namespaces without sharing Rivet's mutable backend registry
• synchronize the CI guide with the pinned Ubuntu 24.04 runner and remove a stale State representation comment

Validation

• raco test tests/ — 8,947 tests passed
• focused Keel/codegen/documentation tests — 326 tests passed
• Taskboard Swift/C++ generated files remained byte-for-byte unchanged
• Scribble documentation built successfully
• raco setup --check-pkg-deps rivet
• git diff --check