Backtest branch's unknown-feature guard was dead code: the if block's
{:error, _} return was discarded and execution fell through, running the
backtest with a just-created atom and writing a report for nonsense
features. Two compounding issues:
- function_exported?/3 returns false for modules not yet loaded in the
BEAM, so even valid feature names triggered the warning path on a
cold VM.
- String.to_atom/1 on caller-supplied strings created unbounded atoms.
Fix: Code.ensure_loaded?(Features) before the check, String.to_existing_atom/1
with rescue (Features exports __info__(:functions) so every valid function
atom already exists), and an explicit early return via a with pipeline so
the error tuple actually leaves the worker.
