feat: add CST-to-AST converter

Single-pass recursive walk that transforms tree-sitter CST
into Go-native AST nodes. This is the only code that touches
tree-sitter after parsing.

Key transformations:
- Delegate chain collapsing (expr->ternary->...->int => LitInt)
- Leaf value pre-parsing (ints, floats, bools at conversion)
- String escape resolution using delimiter context
- Compound assign desugaring (x += 3 => Assign + OpBinary)
- Incr/decr desugaring (x++ => Assign + OpBinary)
- Operator resolution to enum (no string comparison at eval)
- Eager function body conversion (not lazy at call time)
- VarPath flat segment construction from indexed_expr
- Interpolation format specifier extraction

Author: amterp