Replace _Signal class (Python) and SxSignal constructor (JS) with plain
dicts keyed by "__signal". Nine platform accessor functions become ~20
lines of pure SX in signals.sx. type-of returns "dict" for signals;
signal? is now a structural predicate (dict? + has-key?).
Net: -168 lines platform, +120 lines SX. Zero platform primitives for
reactivity — signals compile to any host via the bootstrappers.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>