time: use Numeric#divmod to create Fluent::EventTime instance

[Watson1978]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:
This PR slightly improves the performance of creating Fluent::EventTime instances by replacing separate / and % operations with a single divmod call.

When run rake benchmark:run:in_tail with 10 GB data, here is the result of before/after changing:

	before	after
rake benchmark:run:in_tail	43.305892	43.170137

Microbenchmark

require 'bundler/inline'
gemfile do
  source 'https://rubygems.org'
  gem 'benchmark-ips'
end

Benchmark.ips do |x|

  now = Process.clock_gettime(Process::CLOCK_REALTIME, :nanosecond)

  x.report("div + mod") {
    now / 1_000_000_000
    now % 1_000_000_000
  }

  x.report("divmod") {
    now.divmod(1_000_000_000)
  }

  x.compare!
end

• result

ruby 4.0.1 (2026-01-13 revision e04267a14b) +PRISM [x64-mingw-ucrt]
Warming up --------------------------------------
           div + mod   769.369k i/100ms
              divmod     1.053M i/100ms
Calculating -------------------------------------
           div + mod      7.356M (± 3.6%) i/s  (135.94 ns/i) -     36.930M in   5.027199s
              divmod     10.262M (± 5.5%) i/s   (97.45 ns/i) -     51.612M in   5.045684s

Comparison:
              divmod: 10262087.7 i/s
           div + mod:  7356170.0 i/s - 1.40x  slower

Docs Changes:
N/A

Release Note:

• core: Improve performance of Fluent::EventTime creation using divmod

[kenhys]

LGTM.