Revert to .js extension for optimized files

Editor assessment

`refactor: revert to .js extension for optimized files` records the reversal but not the reason — what the alternative extension broke — so the interesting half of the story is missing. Refactor is defensible for a no-behavior-intent adjustment, and the subject is legible alone. No body supplies the motivation or the history of the prior change, which is exactly the context that would make this instructive. Style is clean: imperative, period-free, no junk. Valid and understandable, but as a teaching example it is a bare mechanical fact.