Reduce Nesting

Deep nesting hides bugs in the branches you didn't read.

• Guard clauses — handle the invalid/edge cases first and return early. The happy path drops to the left margin.
• Invert conditions — if (!valid) return; instead of wrapping the whole body in if (valid) {...}.
• Extract — a nested block doing one thing becomes a named function.
• Replace flag-then-branch — return the result directly instead of setting a variable to return later. Target: no function deeper than 2-3 levels. If you still need more, the function is doing too much — split it.