matrix: widen sync event window backwards — sender precedes type marker

src/matrix.zig

@@ -121,8 +121,11 @@ pub fn sync(alloc: std.mem.Allocator, io: Io, token: []const u8, since: []const
     var i: usize = 0;
     while (std.mem.indexOfPos(u8, resp, i, "\"m.room.message\"")) |pos| {
         defer i = pos + 15;
+        // Event layout: sender (and event_id) come BEFORE the type marker,
+        // body comes after it in content — window spans both sides.
+        const win_start = pos -| 600;
         const win_end = @min(resp.len, pos + 1500);
-        const win = resp[pos..win_end];
+        const win = resp[win_start..win_end];
         const sender = letta.jsonStr(alloc, win, 0, "sender") orelse continue;
         const body = letta.jsonStr(alloc, win, 0, "body") orelse {
             alloc.free(sender.val);