feat(fabric): coalesce a split agent turn into ONE message (deterministic)

OpenClaw delivers an agent turn whose blocks are text -> thinking/tool
-> text via multiple inbound deliver() calls (a non-text block is a
delivery boundary), so one turn became N Fabric messages.

Fix: buffer deliver() segments per channel (src/coalesce.ts) and flush
them as ONE postMessage at a deterministic boundary — the finally after
dispatchInboundReplyWithBase() resolves, which provably runs only after
every deliver() of the turn (verified: deliver,deliver -> dispatch
returned -> flush). No hooks, no timers, no idle guessing. The
agent_end hook was rejected: it fires BEFORE deliver(). gateway_stop
flushes any leftover; a long safety timeout is a leak-guard only.
channels.fabric.coalesce=false restores raw per-segment posting.

Verified on local openclaw + Fabric with a fake text/thinking/text
model: single trigger -> exactly one merged message.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/coalesce.ts

@@ -0,0 +1,93 @@
+// Deterministic turn coalescer.
+//
+// OpenClaw calls the Fabric `deliver` callback once per assistant text
+// segment; a thinking/tool block between two text blocks is a delivery
+// boundary, so one agent turn of `text → thinking/tool → text` arrives as
+// multiple deliver() calls. There is no turn id on the delivery, so we
+// BUFFER segments by Fabric channelId and post the merged message when the
+// turn truly ends. The flush is driven by inbound.ts right after
+// `dispatchInboundReplyWithBase` resolves — that only happens AFTER every
+// deliver() of the turn, a deterministic boundary (NOT a timer, NOT the
+// agent_end hook, which fires before deliver()). `coalesce=false` posts
+// each segment immediately.
+
+const SAFETY_FLUSH_MS = 120_000; // leak-guard only; not the flush mechanism
+
+export function normChannelId(x: string | null | undefined): string {
+  const s = String(x ?? '');
+  return s.startsWith('fabric:') ? s.slice('fabric:'.length) : s;
+}
+
+type Pending = {
+  parts: string[];
+  post: (text: string) => Promise<void>;
+  log?: (m: string) => void;
+  safety: ReturnType<typeof setTimeout>;
+};
+const pendingByChannel = new Map<string, Pending>();
+
+async function flushChannel(channelId: string, reason: string): Promise<void> {
+  const p = pendingByChannel.get(channelId);
+  if (!p) return;
+  pendingByChannel.delete(channelId);
+  clearTimeout(p.safety);
+  const text = p.parts.join('\n\n').trim();
+  if (!text) return;
+  try {
+    await p.post(text);
+    p.log?.(`fabric: flushed ${p.parts.length} segment(s) channel=${channelId} (${reason})`);
+  } catch (e) {
+    p.log?.(`fabric: flush FAILED channel=${channelId} (${reason}): ${String(e)}`);
+  }
+}
+
+// Buffer one delivered segment (or send immediately when coalesce=false).
+// `post` performs the real Fabric postMessage with the caller's already
+// resolved guild/token; on flush it is called once with the merged text.
+export async function enqueueDelivery(params: {
+  channelId: string;
+  text: string;
+  coalesce: boolean;
+  post: (text: string) => Promise<void>;
+  log?: (m: string) => void;
+}): Promise<void> {
+  const cid = normChannelId(params.channelId);
+  const text = (params.text ?? '').trim();
+  if (!text) return;
+  if (!params.coalesce) {
+    await params.post(text);
+    return;
+  }
+  const existing = pendingByChannel.get(cid);
+  if (existing) {
+    existing.parts.push(text);
+    existing.post = params.post; // freshest guild/token closure
+    existing.log = params.log;
+  } else {
+    pendingByChannel.set(cid, {
+      parts: [text],
+      post: params.post,
+      log: params.log,
+      safety: setTimeout(
+        () => void flushChannel(cid, 'safety-timeout'),
+        SAFETY_FLUSH_MS,
+      ),
+    });
+  }
+}
+
+// Called by the agent_end hook with the hook ctx's channelId (bare or
+// fabric:-prefixed). Deterministic per-turn boundary.
+export async function flushFabricForChannel(
+  rawChannelId: string | null | undefined,
+): Promise<void> {
+  const cid = normChannelId(rawChannelId);
+  if (cid) await flushChannel(cid, 'dispatch-end');
+}
+
+// gateway_stop: flush anything still buffered.
+export async function flushAllFabric(): Promise<void> {
+  for (const cid of [...pendingByChannel.keys()]) {
+    await flushChannel(cid, 'gateway_stop');
+  }
+}