fix(storage): floor write-option timestamps in Obsidian adapters

Obsidian's mobile storage layer forwards mtime/ctime to Capacitor's
Filesystem.setTimes, whose native binding casts the value to a Java Long.
A non-integer (float) timestamp makes that cast throw (ClassCastException:
Double cannot be cast to Long), which crashes the app on launch as soon as
such a document is replicated in.

Float timestamps can reach the database from any client that stores
fs.Stats.mtimeMs without flooring. Coerce mtime/ctime to integer ms at the
Obsidian vault and storage adapter write boundary, so a float already
present in the mesh can't crash the app regardless of where it came from.

The truly central choke point is dbToStorage in livesync-commonlib; a
matching guard there would cover the CLI and webapp too. This change
protects the platform that actually crashes.

Claude-Session: https://claude.ai/code/session_0123E9jVQrsgu3zb82Csuwhi

src/serviceModules/FileSystemAdapters/sanitizeWriteOptions.ts

@@ -0,0 +1,25 @@
+import type { UXDataWriteOptions } from "@lib/common/types";
+
+/**
+ * Coerce the timestamp fields of a write-options object to integer milliseconds.
+ *
+ * On mobile, Obsidian forwards `mtime`/`ctime` to Capacitor's
+ * Filesystem.setTimes, whose native binding casts the value to a Java `Long`.
+ * A non-integer (float) timestamp makes that cast throw
+ * `ClassCastException: Double cannot be cast to Long`, which crashes the app on
+ * launch as soon as such a document is replicated in. Float timestamps can
+ * enter the database from any client that stores `fs.Stats.mtimeMs` without
+ * flooring. Flooring at the storage boundary guarantees every Obsidian write
+ * carries an integer, so a float timestamp already present in the mesh cannot
+ * brick the app.
+ *
+ * Returns a shallow copy so the caller's options object is not mutated; passes
+ * `undefined` through unchanged.
+ */
+export function toIntegerTimestamps(options?: UXDataWriteOptions): UXDataWriteOptions | undefined {
+    if (!options) return options;
+    const sanitized: UXDataWriteOptions = { ...options };
+    if (typeof sanitized.mtime === "number") sanitized.mtime = Math.floor(sanitized.mtime);
+    if (typeof sanitized.ctime === "number") sanitized.ctime = Math.floor(sanitized.ctime);
+    return sanitized;
+}