I feel like Python-style indentation should be much easier to parse intuitively (preprocess the line, count leading levels of indentation) than by fully committing to formal theory. Not theoretically optimal and not "single-pass" but is that really the bottleneck?

Yeah, that’s fair. Conceptually it’s not that hard if you’re willing to do a proper preprocess pass and generate INDENT and DEDENT tokens. For this first version I mostly optimized for not shooting myself in the foot, braces gave me very explicit block boundaries, simpler error handling, and a much easier time while bringing up the compiler and codegen. Significant whitespace is definitely interesting long term, but for a v0 learning project I wanted something boring and robust first. Once the core stabilizes, revisiting indentation based blocks would make a lot more sense