Stored timestamps have one writer: local::iso and local::now

The store's time format (RFC 3339, UTC, milliseconds, Z) is what makes lexical
order chronological. It was spelled out ten times as
to_rfc3339_opts(SecondsFormat::Millis, true), with two private now() copies
(store, pull). local::iso(t) and local::now() now hold it; store, pull, engine
and portable call them.

DRY pass #2, batch 2, F5 (#5372).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

core/src/sync/pull.rs

@@ -6,7 +6,6 @@
 //! transaction** as the page it describes — a cursor committed ahead of its data
 //! would silently skip those rows forever, which reads as a clean sync.
 
-use chrono::{SecondsFormat, Utc};
 use rusqlite::{params, Connection, OptionalExtension};
 use serde::Serialize;
 
@@ -14,7 +13,7 @@ use super::blobs::BlobStore;
 use super::client;
 use super::state;
 use super::wire;
-use crate::local::Db;
+use crate::local::{now, Db};
 
 /// Backstop against a server that never stops saying `has_more`. At the server's
 /// 1000-row page cap this is 10M rows — far past any real store, so hitting it means
@@ -108,10 +107,6 @@ async fn download_missing_blobs(
     Ok((downloaded, failed))
 }
 
-fn now() -> String {
-    Utc::now().to_rfc3339_opts(SecondsFormat::Millis, true)
-}
-
 /// Apply one page and advance the cursor, atomically.
 ///
 /// Labels are applied before notes so a membership never references a label row that