Class-based reactivity

Svelte 5 allows $state / $derived / $effect as class fields; Hibiki::Reactive is the Ruby analogue. Declare signals with class macros and use them as plain attributes — no more .value in code:

class Counter
  include Hibiki::Reactive

  state :count, 0
  state(:history) { [] }          # block form: fresh default per instance
  derived(:doubled) { count * 2 }
  effect { puts "count is now #{count}" } # starts on Counter.new

  def increment = self.count += 1
end

counter = Counter.new  # prints "count is now 0"
counter.increment      # prints "count is now 1"
counter.doubled        # => 2

Signals are per-instance and created lazily; subclasses inherit all declarations. Use the block form for mutable defaults (a positional default is one shared object, the same gotcha as Rails attribute defaults) — the Mutable state defaults reference walks through the failure modes and the correct update patterns.