Add scripts/slint-dev-fast.sh: a DEBUG (non-release) build for quick visual
iteration — opt-level 0 so the giant Slint-generated module skips the heavy
LLVM passes, plus the mold linker, no debuginfo, and the nightly parallel
frontend. It builds into target/debug, separate from the release cache.
slint-dev.sh (release, kept for perf measurement) gains the mold linker and
bumps the frontend threads 8 -> 16 — both only cut compile time, the optimised
binary is unchanged.
cc
