fix(base): a queue drain allocates nothing - two lists, reused

The rest of the leak Physics found: a drain that carried facts gave its list away and made a new
one, every queue every frame something happened. The queue keeps two lists and hands one out while
the other fills; a drained list is good until the next drain of that queue (nothing in the game or
the packages keeps one past it). With ab34f82's empty drain and in-place clear, a queue allocates
nothing in steady state. queue_test holds the reuse; every package (403) and lab/unit (90) pass.

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

packages/ludic.base/queue.ludic

@@ -4,6 +4,7 @@ export property Queue<T> {
   items: []T = null
   tag: QueueTag = null
   none: []T = null            # what draining an empty queue hands back, never pushed to
+  spare: []T = null           # the list the last drain handed out: cleared and reused by the next
 }
 # what a queue says about itself, whatever it holds: its name and how many facts wait in it
 export property QueueTag {
@@ -24,6 +25,7 @@ export function queue_new<T>(name: string) -> Queue<T> {
   q.tag = new QueueTag
   q.tag.name = name
   q.none = new []T
+  q.spare = new []T
   return q
 }
 
@@ -32,13 +34,16 @@ export function q_push<T>(q: Queue<T>, v: T) -> void {
   q.tag.pending = len(q.items)
 }
 
-# the facts in the order they were pushed; the queue is empty afterwards. Most queues are empty
-# most frames, and Ludic frees nothing: an empty drain hands back one shared empty list rather
-# than a new one a frame. Read what a drain returns; never push to it.
+# the facts in the order they were pushed; the queue is empty afterwards. Ludic frees nothing, so
+# a drain allocates nothing: the queue keeps two lists and hands one out while the other fills, and
+# an empty drain hands back one shared empty list. What a drain returns is good until the NEXT
+# drain of the same queue - read it, never push to it, never keep it.
 export function q_drain<T>(q: Queue<T>) -> []T {
   if len(q.items) == 0 { return q.none }
   let out = q.items
-  q.items = new []T
+  List.clear(q.spare)
+  q.items = q.spare
+  q.spare = out
   q.tag.pending = 0
   return out
 }

packages/ludic.base/tests/queue_test.ludic

@@ -87,6 +87,23 @@ program QueueTest {
   }
 
   test "a queue drains in the order it was pushed" () { fifo_case() }
+  # two lists, reused: a drained list is the caller's until the next drain of that queue
+  function reuse_case() -> void {
+    let q: Queue<int> = queue_new("reused")
+    q_push(q, 1)
+    let a = q_drain(q)
+    q_push(q, 2)
+    expect_eq(a[0], 1)
+    let b = q_drain(q)
+    expect_eq(b[0], 2)
+    q_push(q, 3)
+    let c = q_drain(q)
+    expect(c == a)
+    expect_eq(len(c), 1)
+    expect_eq(c[0], 3)
+  }
+
+  test "a drain reuses the queue's two lists" () { reuse_case() }
   test "an empty drain allocates nothing, and a drained list is the caller's" () { empty_case() }
   test "a queue holds records" () { record_case() }
   test "clear drops what is waiting" () { clear_case() }