fix(wake): #908 unify observed_seq on a single store-side allocator (dissolve detector-private-counter seam) (#915)

Co-authored-by: jason.woltje <jason@diversecanvas.com>
Co-committed-by: jason.woltje <jason@diversecanvas.com>

packages/mosaic/framework/tools/wake/_wake-common.sh

@@ -65,6 +65,51 @@ _atomic_write() {
   fi
 }
 
+# ---------------------------------------------------------------------------
+# Enqueue serialization (single store-side observed_seq allocator, #908).
+#
+# store.sh is the SOLE allocator of observed_seq: it reads the observed_seq
+# cursor, computes next = observed_seq + 1, and writes the pending record +
+# observed.set + the cursor as ONE transaction. That read-modify-write MUST be
+# serialized so two concurrent enqueues cannot read the same cursor and collide
+# on a seq. These helpers take an exclusive lock for the duration of the
+# transaction. flock (Linux/CI) is the load-bearing mechanism; where flock is
+# absent the open still succeeds so a single-threaded enqueue is unaffected (the
+# concurrency guarantee then degrades — the concurrency test SKIPs without flock,
+# exactly as the detector's single-instance test already does).
+#
+# The lock uses a FIXED fd (8) within one store.sh process. Each store.sh
+# invocation is its own process (the detector/reconciler call it as a
+# subprocess), so fd 8 is always free here and never clashes with the detector
+# run-loop lock (fd 9, a DIFFERENT process).
+# ---------------------------------------------------------------------------
+_wake_lock_acquire() {
+  # _wake_lock_acquire LOCKFILE — open fd 8 on LOCKFILE and take an exclusive
+  # (blocking) lock. Returns non-zero if the lock cannot be taken.
+  local lf="$1" dir
+  dir="$(dirname "$lf")"
+  [ -d "$dir" ] || mkdir -p "$dir" 2>/dev/null || true
+  # Open the lock fd. If the file does not yet exist and the dir is writable it
+  # is created; if the dir is read-only but the file exists, opening it O_WRONLY
+  # still succeeds (write perm on the file, not the dir).
+  # NB: a command-less `exec` redirection persists for the WHOLE shell, so we must
+  # NOT append `2>/dev/null` here (it would permanently silence the caller's
+  # stderr and swallow every later fail-loud diagnostic). Redirect only fd 8.
+  exec 8>"$lf" || return 1
+  if command -v flock >/dev/null 2>&1; then
+    flock 8 || return 1
+  fi
+  return 0
+}
+
+_wake_lock_release() {
+  # _wake_lock_release — drop the enqueue lock (closing fd 8 releases the flock).
+  # The `2>/dev/null` is SCOPED to the brace group (suppressing a "bad fd" close
+  # error) — it must NOT sit on a bare `exec`, where the redirection would persist
+  # for the whole shell and silence every later fail-loud diagnostic.
+  { exec 8>&-; } 2>/dev/null || true
+}
+
 # _wake_clean_stale_tmp DIR — remove leftover atomic-write temp files (e.g. from
 # a crash mid-write). Safe: these are never the live store.
 _wake_clean_stale_tmp() {