LlEmitter's field list and constructor are where most concurrent edits to
emitter.ts collide. A feature now defines its state in its own file with
defineModuleState (emitter-state.ts) and reads it through the returned key,
so it no longer edits the class. The loop optimizations' twelve fields and
their per-function setup move to loop-state.ts; the emitted IR is unchanged.
scripts/split holds the transforms used for the emitter and lowering
splits, and rehome-split.mjs, which carries an open branch across a split.
Chris Tate