Don't assert on a timer started during dispatch

[nob13]

aml_dispatch() reads now once, drains what is due, then asserts the earliest
remaining deadline is strictly in the future:

uint64_t now = aml__gettime_us(self);
while (aml__handle_timeout(self, now));

struct aml_timer* earliest = aml__get_timer_with_earliest_deadline(self);
if (earliest) {
        assert(earliest->deadline > now);

aml_start_timer() may run on another thread in between, and computes its
deadline from a later aml__gettime_us(). So the invariant is >= now, not
> now. A zero timeout collides within the same microsecond and hits it easily.

Repro: 2000 zero-timeout timers from a foreign thread against a busy loop aborts
3 of 5 runs before, 0 of 5 after.