Add reactive islands spec: signals.sx + defisland across all adapters

New spec file signals.sx defines the signal runtime: signal, computed,
effect, deref, reset!, swap!, batch, dispose, and island scope tracking.

eval.sx: defisland special form + island? type predicate in eval-call.
boundary.sx: signal primitive declarations (Tier 3).
render.sx: defisland in definition-form?.
adapter-dom.sx: render-dom-island with reactive context, reactive-text,
  reactive-attr, reactive-fragment, reactive-list helpers.
adapter-html.sx: render-html-island for SSR with data-sx-island/state.
adapter-sx.sx: island? handling in wire format serialization.
special-forms.sx: defisland declaration with docs and example.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/boundary.sx

@@ -124,3 +124,55 @@
 (define-boundary-types
   (list "number" "string" "boolean" "nil" "keyword"
         "list" "dict" "sx-source"))
+
+
+;; --------------------------------------------------------------------------
+;; Tier 3: Signal primitives — reactive state for islands
+;;
+;; These are pure primitives (no IO) but are separated from primitives.sx
+;; because they introduce a new type (signal) and depend on signals.sx.
+;; --------------------------------------------------------------------------
+
+(declare-tier :signals :source "signals.sx")
+
+(declare-signal-primitive "signal"
+  :params (initial-value)
+  :returns "signal"
+  :doc "Create a reactive signal container with an initial value.")
+
+(declare-signal-primitive "deref"
+  :params (signal)
+  :returns "any"
+  :doc "Read a signal's current value. In a reactive context (inside an island),
+        subscribes the current DOM binding to the signal. Outside reactive
+        context, just returns the value.")
+
+(declare-signal-primitive "reset!"
+  :params (signal value)
+  :returns "nil"
+  :doc "Set a signal to a new value. Notifies all subscribers.")
+
+(declare-signal-primitive "swap!"
+  :params (signal f &rest args)
+  :returns "nil"
+  :doc "Update a signal by applying f to its current value. (swap! s inc)
+        is equivalent to (reset! s (inc (deref s))) but atomic.")
+
+(declare-signal-primitive "computed"
+  :params (compute-fn)
+  :returns "signal"
+  :doc "Create a derived signal that recomputes when its dependencies change.
+        Dependencies are discovered automatically by tracking deref calls.")
+
+(declare-signal-primitive "effect"
+  :params (effect-fn)
+  :returns "lambda"
+  :doc "Run a side effect that re-runs when its signal dependencies change.
+        Returns a dispose function. If the effect function returns a function,
+        it is called as cleanup before the next run.")
+
+(declare-signal-primitive "batch"
+  :params (thunk)
+  :returns "any"
+  :doc "Group multiple signal writes. Subscribers are notified once at the end,
+        after all values have been updated.")