Use bigint for paths to avoid PathTooLarge errors

[Rigidity]

Note

Medium Risk
Touches core lowering and CLVM path codegen for every symbol reference; behavior should match for normal depths but large-path edge cases depend on the new BigUint math and atom encoding.

Overview
CLVM environment paths are now represented with BigUint end-to-end instead of u32, so deep binding trees no longer fail with PathTooLarge.

Environment::path builds paths with left-shift/add instead of checked_mul, and PathError::PathTooLarge is removed (only SymbolNotFound remains). Lir::Path and lowering/codegen/optimizer call sites use BigUint; path atoms go through path_to_atom with the same big-endian trimming behavior, extended for large values.

Path math in path.rs (first_path, rest_path, parent_path) is rewritten for arbitrary size, with a new test that walks 100 rest/parent steps.

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