fix(sync): hold a drained cursor a minute back so late commits arrive

A page's watermark is its last row's stamp, and a long import stamps its rows before it commits them: a quick write committing in between carried the watermark past them, and the keyset never returned them. A caught-up page now hands out watermarks at most a minute before serverNow in every row domain, as start=now does, so the next pull reads that minute again. The mirror domain keeps its watermark.

src/lib/sync/changes/pull.ts

@@ -14,6 +14,7 @@ import {
   decodeCursorIssuedAt,
   encodeCursor,
   encodeCursorAtNow,
+  holdBackDrainedCursor,
   SYNC_DOMAINS,
   type SyncCursor,
   type SyncDomain,
@@ -232,6 +233,7 @@ export async function pullSyncChanges(input: {
     cd.hasMore ||
     cy.hasMore ||
     mm.hasMore;
+  if (!hasMore) holdBackDrainedCursor(nextCursor, serverNow.getTime());
 
   annotate({
     action: { name: "sync.changes.pull" },

src/lib/sync/cursor.ts

@@ -129,6 +129,34 @@ export function encodeCursorAtNow(nowMs: number): string {
   return encodeCursor(cursor, nowMs);
 }
 
+/**
+ * Hold a drained walk's watermarks back to `START_NOW_OVERLAP_MS` before
+ * `nowMs`, in place. The watermark of a page is the stamp of its last row,
+ * and that stamp says nothing about rows still in flight: a long write (a
+ * Garmin or Withings chunk holds its transaction for seconds) stamps its rows
+ * first and commits them last, so a quick write that commits in between can
+ * carry the watermark past them, and the keyset would never return them.
+ * Held back a minute, the next pull reads the last minute again and the
+ * late rows come with it; anything older is passed as before. Only the
+ * cursor of a walk that is done is held back: one that still has pages keeps
+ * moving forward, or a minute holding more than a page could never drain.
+ *
+ * `measurementMirror` is left alone. Its watermark is a token's mirror
+ * stamp, written by one short update, and reading its last minute again
+ * would send every reading of a token whose mirror switch just flipped
+ * once more on every pull in that minute.
+ */
+export function holdBackDrainedCursor(cursor: SyncCursor, nowMs: number): void {
+  const ceiling = nowMs - START_NOW_OVERLAP_MS;
+  for (const domain of SYNC_DOMAINS) {
+    if (domain === "measurementMirror") continue;
+    const wm = cursor[domain];
+    if (wm && wm.updatedAtMs > ceiling) {
+      cursor[domain] = { updatedAtMs: ceiling, id: "0" };
+    }
+  }
+}
+
 function parseWatermark(raw: unknown): DomainWatermark | null {
   if (typeof raw !== "object" || raw === null) return null;
   const u = (raw as { u?: unknown }).u;