feat(wake): W6 — off-host dead-man beacon + pluggable alarm-sink adapter

A8 of the wake canon (EPIC #892, W6): the independent liveness leg.

beacon.sh:
- MONOTONIC EMITTER (emit): strictly-increasing seq + emit_ts each cycle,
  shipped off-host via a pluggable sink adapter. The primitive the detector
  run-loop calls each poll (§1.3).
- DEAD-MAN / ABSENCE alarm (record + check): the off-host monitor's receiver
  and absence check. A missing/stale-past-SLO beacon FIRES the alarm and ROUTES
  it to a human/other-host, depending on nothing the dying host does (§1.3/§4-G1).
- PLUGGABLE alarm-sink / beacon-sink ADAPTER INTERFACE: the framework ships the
  emitter + interface; the operator owns the target endpoint, resolved BY NAME
  (load_credentials), never inlined (§1.4). FAIL-CLOSED: unconfigured OR
  unreachable target FAILS LOUD — no silent no-alarm host (§4-G2a).
- DEGRADED MODES (honest, not silent): a same-host sibling is REJECTED as
  non-independent (shares user-manager/host/sender/socket/session); an isolated
  host degrades to a FLAGGED different-supervision-root beacon; capture-pane is
  a liveness HINT only (§1.3).

detector.sh: a single minimal, opt-in W6 emit seam in the run-loop (inert unless
the operator wires WAKE_BEACON_SINK_CMD; a beacon emit failure never kills the
loop but is loud). No store/ack/digest/sign code touched.

test-wake-beacon.sh: RED-FIRST harness (10 invariant groups), wired into
test:framework-shell. manifest.txt bumped 0.4.0 -> 0.5.0 + inventory.

Part of #892

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0158NZqN2n2ymKFeJAZ4GUCb

packages/mosaic/framework/tools/wake/manifest.txt

@@ -15,8 +15,10 @@
 #   0.2.0  W3 — cumulative-state digest renderer + non-circular HMAC signer.
 #   0.3.0  W4 — per-host single-instance delta-gated detector daemon.
 #   0.4.0  W5 — synthetic-canary FN-oracle + source-parity reconciler.
+#   0.5.0  W6 — off-host dead-man beacon emitter + pluggable alarm-sink adapter
+#              + beacon-absence alarm (fail-loud on unconfigured/unreachable).
 component=wake
-version=0.4.0
+version=0.5.0
 
 # Watch-list schema this component consumes, and the INCLUSIVE range of
 # schema_version values it supports. A wake-watch-list.json whose schema_version
@@ -45,4 +47,16 @@ schema_max=1
 #                    inventory (an omitted source cannot pass the vector
 #                    vacuously) + (ii) periodic full reconcile to 0-unaccounted,
 #                    enumerating pre-existing/startup state into the store.   (W5)
-# Out of scope (later waves): off-host beacon (W6), installer (W7).
+#   beacon.sh   A8 — off-host DEAD-MAN liveness beacon: a monotonic beacon
+#                    EMITTER (emit — the primitive the detector run-loop calls
+#                    each cycle), the off-host monitor's RECEIVER + beacon-ABSENCE
+#                    alarm (record, check), and a pluggable alarm-sink/beacon-sink
+#                    ADAPTER INTERFACE. Liveness is SPLIT from work-triggering;
+#                    the alarm fires on ABSENCE, routing to a human/other-host
+#                    within its SLO (§4/G1). FAIL-CLOSED: an unconfigured OR
+#                    unreachable target FAILS LOUD (no silent no-alarm host).
+#                    A same-host sibling is REJECTED as non-independent; an
+#                    isolated host degrades to a FLAGGED different-supervision-root
+#                    beacon; capture-pane is a liveness HINT only.            (W6)
+# Out of scope (later waves): installer (W7 wires + install-validates the beacon
+# target that beacon.sh's fail-loud primitive is designed for).