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/datetime.zig

@@ -502,6 +502,28 @@ pub const DateTime = struct {
     }
 };
 
+pub fn timestamp() u32 {
+    const ts = timespec();
+    return @intCast(ts.sec);
+}
+
+pub fn milliTimestamp() u64 {
+    const ts = timespec();
+    return @as(u64, @intCast(ts.sec)) * 1000 + @as(u64, @intCast(@divTrunc(ts.nsec, 1_000_000)));
+}
+
+fn timespec() std.posix.timespec {
+    const clock_id = switch (@import("builtin").os.tag) {
+        .freebsd, .dragonfly => std.posix.CLOCK.MONOTONIC_FAST,
+        .macos, .ios, .tvos, .watchos, .visionos => std.posix.CLOCK.UPTIME_RAW, // continues counting while suspended
+        .linux => std.posix.CLOCK.BOOTTIME, // continues counting while suspended
+        else => std.posix.CLOCK.MONOTONIC,
+    };
+    // we don't currently support platforms where, at the very least,
+    // posix.CLOCK.MONOTONIC wouldn't be available.
+    return std.posix.clock_gettime(clock_id) catch unreachable;
+}
+
 fn writeDate(into: []u8, date: Date) u8 {
     var buf: []u8 = undefined;
     // cast year to a u16 so it doesn't insert a sign