askrene-migration: contrib script robustness + de-flake coalesce test

Three fixes from the 2026-06-29 CodeRabbit pass on PR #320.

clboss-askrene-layer-summary: load_layers() now catches OSError and
JSONDecodeError on the --input replay path and exits with a message
instead of crashing with a traceback on a missing file or malformed
JSON. This is the offline mode the help text advertises.

clboss-askrene-layer-summary: reject --json together with --top. The
combination previously printed a JSON object and then appended a
human-readable top section, producing invalid mixed-format output.
The top list is human-readable only.

test_askrene_layer: pin the coalescing bucket window astronomically
large in test_coalesce_drops_dominated so both writes always land in
the same Ev::now()/window bucket. Previously a run straddling a bucket
boundary re-emitted the dominated write as a keep-alive and failed the
sentinel assertion. It is the final test before Shutdown, so the global
window needs no restore.

contrib/clboss-askrene-layer-summary

@@ -128,8 +128,15 @@ def percentile(sorted_vals, q):
 
 def load_layers(args, network_option):
     if args.input is not None:
-        raw = sys.stdin.read() if args.input == "-" else open(args.input).read()
-        return json.loads(raw).get("layers", [])
+        try:
+            if args.input == "-":
+                raw = sys.stdin.read()
+            else:
+                with open(args.input) as f:
+                    raw = f.read()
+            return json.loads(raw).get("layers", [])
+        except (OSError, json.JSONDecodeError) as e:
+            sys.exit(f"failed to load --input data: {e}")
     res = run_lightning_cli_command(
         args.lightning_dir, network_option, "askrene-listlayers", args.layer
     )
@@ -239,6 +246,10 @@ def main():
     s = summarize(target)
     depth = s.pop("_depth")
 
+    if args.json and args.top > 0:
+        sys.exit("--json cannot be combined with --top "
+                 "(the top list is human-readable only)")
+
     if args.json:
         print(json.dumps(s, indent=2))
     else:

tests/boss/test_askrene_layer.cpp

@@ -320,6 +320,16 @@ test_coalesce_drops_dominated( MockRpcServer& server
 			     ) {
 	auto const layer = std::string("coalesce-layer");
 
+	/* Pin the coalescing bucket astronomically large so both writes
+	 * below always land in the same Ev::now()/window bucket regardless
+	 * of when the test runs; otherwise a run straddling a bucket
+	 * boundary re-emits the dominated write as a keep-alive and
+	 * assert_sentinel fires spuriously.  This is the final test before
+	 * Shutdown, so the global window needs no restore. */
+	Boss::Mod::AskreneLayer::set_aging_window_secs(
+		std::uint64_t(1000) * 365 * 24 * 60 * 60
+	);
+
 	auto assert_first = [](Jsmn::Object const& req) {
 		auto id = assert_method(req, "askrene-inform-channel");
 		auto params = req["params"];