feat(server): add builtin function typechecking + BuiltinFn enum

[Khanathan]

Summary

First step of RFC-001: Pythonic expressions. Converts BuiltinFn from a
static struct table to a closed Rust enum, promotes cast to its own
Expr::Cast AST node, and switches Expr::Call to carry the resolved
BuiltinFn instead of a String. Adds shared type-inference primitives +
helpers for later components. No IR or Python change.

Linked issues

Linked to #56

Implementation Plan

• Add tests for all components that will be implemented in this PR
• Move expr_builtins.rs → builtins/mod.rs; update 5 importers. Old built-in tests still pass
• Add InferError and TypeClass.
• Add 4 reusable typechecking primitives + 8 shared helpers. All tests for those pass
• BuiltinFn enum (variants: IsNull, Quadkey — Cast excluded) + 5 match self methods + Display + name↔variant round-trip. Replaces old struct, BUILTINS slice, and lookup_builtin
• quadkey_infer (the one builtin without a shared helper). Project builds cleanly
• Refactor Expr::Call.fn_name: String → Expr::Call.builtin: BuiltinFn; parser resolves names at parse time and emits ExprParseError::UnknownBuiltin
• Promote cast to Expr::Cast { operand, target, span }; add arms in span, referenced_fields, to_expr_string, eval_depth, infer_expr_type_inner
• Runs all tests and style checks. Everything passes, no warnings

Verification:

All three Rust gates passed:

• fmt: 25s
• clippy: 324s
• test: 587s

Pre-flight checklist

• bash .github/scripts/check.sh exits 0 (or each step passes manually)
• Verification block above replaced with real output