refactor: host Rust-only data models, constants and errors natively

[kvlonge]

Why

Move data models, constants and errors that only Rust-hosted code uses into Rust, while Python keeps only the shapes the executor, adapters and user code consume.

Changes

• Compiler: dead constants and types deleted, Rust-read constants moved to Rust, annotation-only aliases moved to stubs, project assembly facts returned as native classes.
• Planner: constants, value records, enums and fixture column inferences hosted natively; executor-facing plan types stay Python.
• CLI, dbt, cost and freshness: command requests, entry and output models, records, enums and the CLI user error family hosted natively with unchanged equality, repr, pickling and error hierarchy.
• Planner differential compares captured types by class name so natively hosted types still match the baseline.
• Oversized rendered constants still report a compile error.
• Movable Python decreases from 13,066 to 9,602 lines.

Verification

• 11,387 unit/integration tests passed; make check-ci passed with all optional dependencies available.
• Compiler differential: 371 projects, unchanged goldens. Planner differential: 11 projects matched the main baseline.
• Render and discovery capture tests pass.
• Cold and edited compile outputs and exit codes match the reference on synthetic and private compile-only projects.
• Worker CLI captures (including --json and error paths) matched before and after each slice.
• Independent review: one medium finding fixed with a regression test and verified.