fix(jsonl-merge): make equal-ts resolution converge across machines

The JSONL append merge driver sorted timestamped entries by (0, ts) with no
further tiebreaker. Equal-ts entries then fell back to stable-sort insertion
order (base, ours, theirs), but git assigns the local side to "ours", so two
machines resolving the same conflict emitted equal-ts lines in opposite order.
The merged files diverged and never converged. gstack-telemetry-log uses
second-granularity timestamps, so same-ts collisions are routine.

Add the line content as the final sort tiebreaker so the order is total and
side-independent. Add a regression test that runs the driver with the two
sides swapped and asserts identical output.

bin/gstack-jsonl-merge

@@ -53,18 +53,25 @@ for path in paths:
                     continue
                 if line in seen:
                     continue
-                # Prefer ISO ts field for sort; fall back to SHA-256.
+                # Prefer ISO ts field for sort; fall back to SHA-256. The line
+                # content is the final tiebreaker so the order is total: two
+                # entries sharing a ts must resolve identically regardless of
+                # which side they arrive on. Without it, equal-ts entries fall
+                # back to insertion order (base, ours, theirs), and since ours
+                # and theirs are swapped depending on which machine runs the
+                # merge, the two sides produce divergent files that never
+                # converge.
                 sort_key = None
                 try:
                     obj = json.loads(line)
                     ts = obj.get('ts') or obj.get('timestamp')
                     if isinstance(ts, str):
-                        sort_key = (0, ts)
+                        sort_key = (0, ts, line)
                 except (json.JSONDecodeError, ValueError, TypeError):
                     pass
                 if sort_key is None:
                     h = hashlib.sha256(line.encode('utf-8')).hexdigest()
-                    sort_key = (1, h)
+                    sort_key = (1, h, line)
                 seen[line] = sort_key
     except FileNotFoundError:
         # Absent base / absent ours / absent theirs are all valid.