refactor: simplify createWriteCheckpoint Cosmos DB HEAD capture

Replace the polling loop in createWriteCheckpoint with a direct read
of the current storage checkpoint LSN.

The polling loop (get baseline → write sentinel → poll until LSN
advances) was unnecessary: with >= comparison, the poll always resolved
on the first iteration because the baseline LSN was already in storage.
The loop was effectively just reading the current LSN.

The simplified version reads the current checkpoint LSN directly and
uses it as HEAD. This is correct because:
- createReplicationHead already wrote a sentinel to _powersync_checkpoints
- The sentinel guarantees the streaming loop will advance past this point
- The HEAD just needs to be a valid LSN at or before the sentinel
- The sync stream resolves the write checkpoint when replication advances

Removes ~15 lines of polling/timeout code.

Author: Sleepful

packages/service-core/src/util/checkpointing.ts

@@ -20,29 +20,24 @@ export async function createWriteCheckpoint(options: CreateWriteCheckpointOption
     let head = currentCheckpoint;
 
     if (head == null) {
-      // Cosmos DB: HEAD unknown. Poll storage until the streaming loop
-      // processes the sentinel and advances the checkpoint LSN.
-      // On Cosmos DB, wall-clock LSNs have second precision — the sentinel
-      // commit may produce the same LSN as the baseline if both fall in the
-      // same wall-clock second. Use >= (not >) so the poll resolves as soon
-      // as any commit happens, even at the same second. The sentinel write
-      // guarantees the streaming loop will process at least one event.
-      const baselineCheckpoint = await syncBucketStorage.getCheckpoint();
-      const baselineLsn = baselineCheckpoint?.lsn ?? '';
-
-      const timeout = 30_000;
-      const start = Date.now();
-      while (Date.now() - start < timeout) {
-        const cp = await syncBucketStorage.getCheckpoint();
-        if (cp?.lsn && cp.lsn >= baselineLsn) {
-          head = cp.lsn;
-          break;
-        }
-        await new Promise((r) => setTimeout(r, 50));
-      }
-      if (!head) {
-        throw new ServiceError(ErrorCode.PSYNC_S2302, 'Timeout waiting for sentinel checkpoint');
+      // Cosmos DB: operationTime / clusterTime not available on regular
+      // commands, so createReplicationHead cannot capture the HEAD directly.
+      // Instead, use the current storage checkpoint LSN. This is valid because:
+      //
+      // 1. createReplicationHead already wrote a sentinel to _powersync_checkpoints,
+      //    guaranteeing the streaming loop will advance past this point.
+      // 2. The sync stream's watchCheckpointChanges resolves the write checkpoint
+      //    when replication advances past the stored HEAD.
+      // 3. The sentinel ensures forward progress even on an idle system.
+      //
+      // The HEAD doesn't need to be the exact sentinel position — it just needs
+      // to be a valid LSN at or before the sentinel. The current storage LSN
+      // satisfies this because it was committed before the sentinel was written.
+      const cp = await syncBucketStorage.getCheckpoint();
+      if (!cp?.lsn) {
+        throw new ServiceError(ErrorCode.PSYNC_S2302, 'Cannot create write checkpoint: no replication checkpoint available');
       }
+      head = cp.lsn;
     }
 
     const writeCheckpoint = await syncBucketStorage.createManagedWriteCheckpoint({