mailbox S0: dual-glue flag, beacon counters, N2 red spec

Doc 17 step S0 scaffolding: WX_SCHEDULER=1 injector path with an
observation-only asyncify-scheduler.js skeleton (legacy shim stays
authoritative until S2), guard-beacon extraction with occurrence
recovery for rate-limited beacons, and the fixme'd N2 ordering spec
(add-then-move probe; un-fixme at S1).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TfxKn5utcntBSnxz4ZnYKs

scripts/common/shims/asyncify-scheduler.js

@@ -0,0 +1,42 @@
+// === AsyncifyScheduler (S0 scaffolding — observation-only) ===
+// docs/features/async/17-mailbox-scheduler-plan.md · injected only on WX_SCHEDULER=1 builds.
+//
+// S0 contract: this file changes NO runtime behavior. It claims the namespace, the
+// registry data structures, and the build marker so (a) the dual-glue build variant
+// exists and can run the full suite, and (b) tests can detect which runtime they're on.
+// S2 turns this into the sole owner of Asyncify.currData/state (doc 13 §1): the four
+// hooks (handleSleep wrap, fiber-swap tracking, trampoline ownership, deferred drain)
+// land there, gated on the N1 single-writer tripwire. Until then the legacy
+// handlesleep.js shim (injected just above) stays authoritative.
+if (typeof Asyncify !== "undefined" && !globalThis.__wxSchedulerInstalled) {
+  globalThis.__wxSchedulerInstalled = true;
+  Asyncify.__schedulerBuild = 1;
+
+  var AsyncifyScheduler = {
+    // ctx = { id, kind: 'main'|'modal'|'nested'|'coroutine'|'sleep',
+    //         buffer, status: 'running'|'parked'|'ready', wakeReason, result }
+    contexts: new Map(),
+    readyQueue: [],
+    running: null,
+    transitionRunning: false,
+    trampolineRunning: false,
+
+    // S2 fills these in. They throw today so a premature caller is loud, not silent —
+    // nothing in an S0 build calls them.
+    park: function () { throw new Error("[wx-scheduler] park(): not implemented until S2"); },
+    resume: function () { throw new Error("[wx-scheduler] resume(): not implemented until S2"); },
+    drain: function () { throw new Error("[wx-scheduler] drain(): not implemented until S2"); },
+
+    state: function () {
+      return "[wx-scheduler] build=1 impl=S0-observation-only"
+        + " contexts=" + this.contexts.size
+        + " ready=" + this.readyQueue.length
+        + " running=" + this.running
+        + " transition=" + this.transitionRunning;
+    },
+  };
+
+  globalThis.__wxScheduler = AsyncifyScheduler;
+  console.log("[wx-scheduler] scaffolding installed (S0, observation-only)");
+}
+// === End AsyncifyScheduler ===