fix(environment): preserve submillisecond insert ordering

Compare retained submillisecond components after milliseconds, so ordered collections recognize and place distinct timestamps correctly.

Fixes #6525

src/domain/analysisView/environmentStateWriteExplain.test.ts

@@ -553,6 +553,7 @@ it.each<[JsonValue | undefined, JsonValue | undefined, number | undefined]>([
   ["10", 9, 1],
   ["7", 7, 0],
   ["2026-03-04T05:00:00Z", "2026-03-04T06:30:00+02:00", 1],
+  ["2026-03-04T05:00:00.123000001Z", "2026-03-04T05:00:00.123000002Z", -1],
   [Date.UTC(2026, 2, 4, 6) / 1000, "2026-03-04T05:00:00Z", 1],
   ["apple", "banana", -1],
   ["Zebra", "apple", -1],
@@ -583,6 +584,26 @@ it("offers each strictly ordered field in its direction, then last and first", (
   expect(insertCandidates([], ["id"])).toEqual(fallback);
 });
 
+it("uses retained submillisecond precision to recognize and place sorted timestamps", () => {
+  const members = [
+    { at: "2026-03-04T05:00:00.123000002Z" },
+    { at: "2026-03-04T05:00:00.123000003Z" },
+  ];
+  const rule: InsertRule = {
+    rule: "sorted",
+    field: "at",
+    direction: "ascending",
+  };
+  expect(insertCandidates(members, ["at"])).toEqual([
+    rule,
+    { rule: "last" },
+    { rule: "first" },
+  ]);
+  expect(
+    insertIndex(rule, members, { at: "2026-03-04T05:00:00.123000001Z" }),
+  ).toBe(0);
+});
+
 const ASCENDING = [{ n: 1 }, { n: 3 }, { n: 5 }];
 const DESCENDING = [{ n: 5 }, { n: 3 }, { n: 1 }];
 const up: InsertRule = { rule: "sorted", field: "n", direction: "ascending" };

src/domain/analysisView/environmentStateWriteExplain.ts

@@ -431,7 +431,8 @@ export const compareValues = (
   const [a, b] = [integerOf(left), integerOf(right)];
   if (a !== undefined && b !== undefined) return a - b;
   const [c, d] = [readClock(left), readClock(right)];
-  if (c !== undefined && d !== undefined) return c.ms - d.ms;
+  if (c !== undefined && d !== undefined)
+    return c.ms - d.ms || c.subMillisecond - d.subMillisecond;
   return typeof left === "string" && typeof right === "string"
     ? compare(left, right)
     : undefined;