Refactor cfold propagation and add eager mode

[Gustavo2622]

Refactor `cfold` in `ecPhlCodeTx` to track propagated substitutions
and preserved dependencies explicitly, and use that state to decide
when folding can continue across assignments and structured
instructions.

Add an eager `cfold*` variant that keeps folding by promoting
preserved variables into the propagated substitution when possible,
and extend `tests/cfold.ec` with coverage for both the default and
eager behaviors.

[strub]

Fix the compilation warnings (CI is failing)