- beam-init: runs a JS expression once after the state scope is fully
  initialized (useful for setInterval autoplay, timers, derived values)
- src/directives.ts: typed registry of all beam reactivity directives
  with descriptions and usage examples; exports BEAM_EXACT_NAMES,
  BEAM_PREFIX_NAMES, and isValidBeamReactivityDirective() helper
- package.json: add ./directives export entry