Improve clocks

There's a flaky performance test that I wanted to fix (1). This led to a couple
changes.

1 - Add timestamp() and milliTimestamp() to datetime.zig. Reduce some code
    duplication and use better clock_ids where available

2 - Change Performance API to use milliTimestamp and store a u64 instead of a
    f64. While the spec says a float, Firefox deals with u64 and implicit
    conversion is always available. Makes our APIs simpler.

(1) - https://github.com/lightpanda-io/browser/actions/runs/17313296490/job/49151366798#step:4:131

src/log.zig

@@ -321,14 +321,14 @@ fn writeString(comptime format: Format, value: []const u8, writer: anytype) !voi
     return writer.writeByte('"');
 }
 
-fn timestamp() i64 {
+fn timestamp() u64 {
     if (comptime @import("builtin").is_test) {
         return 1739795092929;
     }
-    return std.time.milliTimestamp();
+    return @import("datetime.zig").milliTimestamp();
 }
 
-var first_log: i64 = 0;
+var first_log: u64 = 0;
 fn elapsed() struct { time: f64, unit: []const u8 } {
     const now = timestamp();