Introduce _const_fold, a whitelist-only, bounded, side-effect-free partial
evaluator plus a single-assignment const-prop environment builder. It recomputes
pure transforms on literals only (concat, repeat, join, format, f-strings,
slice/reverse, chr/ord, base64/hex/rot13/zlib decode, pure builtins and string
methods) and never executes, imports, or reflects on user code. Depth, op, size,
and sequence caps guarantee it can only fail to recover a value, never crash or
hang. This is the foundation the later eval/exec unwrapping and filesystem path
resolver build on.
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