feat(lang): 0.R5 - a reducer writes one state and may read others, declared between its state and the action

What only becomes known inside the drain - a value another reducer just set, the map in play - no longer
has to be faked into the action, so a verb that reads several systems while it changes one is a reducer
instead of an act handed its states. A second state to write is still refused, and the message says the
way out. examples/actions/reads.ludic; reseeded.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

examples/actions/reads.ludic

@@ -0,0 +1,23 @@
+# reads.ludic - 0.R5: a reducer writes one state and may READ others, declared between its state and
+# the action: what only becomes known inside the drain (a price another reducer set) need not ride in
+# the action, and still no reducer changes a state it does not own.
+program ReducerReads {
+  state Shop { price: int = 3 }
+  state Wallet { cash: int = 10 }
+  state Pack { n: int = 0 }
+  action Rise { by: int }
+  action Buy { }
+
+  reducer Shop on Rise(s: mut Shop, a: Rise) { s.price += a.by }
+  reducer Wallet on Buy(w: mut Wallet, s: Shop, a: Buy) {
+    if w.cash >= s.price { w.cash -= s.price }
+  }
+  reducer Pack on Buy(p: mut Pack, w: Wallet, a: Buy) { p.n += 1 }
+
+  entry (s: Shop, w: Wallet, p: Pack) {
+    dispatch Rise { by: 2 }
+    dispatch Buy { }
+    drain_actions()
+    print(`{s.price} {w.cash} {p.n}`)
+  }
+}

examples/rejected/reducer_two_states.ludic

@@ -1,8 +1,8 @@
-# 0.R: a reducer takes its own state and the action; what else it needs rides in the action
+# 0.R: a reducer writes its own state and may read others; a second state to write is refused
 program ReducerTwoStates {
   state Pack { n: int = 0 }
   state Wallet { cash: int = 0 }
   action Buy { cost: int }
-  reducer Pack on Buy(p: mut Pack, w: mut Wallet) { p.n += 1 }
+  reducer Pack on Buy(p: mut Pack, w: mut Wallet, a: Buy) { p.n += 1 }
   entry { print(1) }
 }