Add `monotonic_seconds` to the clock interface and rename `monotonic_time`

[bolshakov]

Problem

SlidingWindow exposes a private monotonic_seconds helper that converts @clock.monotonic_time (ms) to seconds:

def monotonic_seconds
  @clock.monotonic_time / 1000.0
end

The ms→s division is a unit-conversion concern that belongs on the clock, not on every caller. monotonic_time also has an ambiguous name — the unit is not apparent from the method name.

Proposed change

• Add monotonic_seconds to the clock interface (SystemClock, NullClock) returning Float seconds via Process.clock_gettime(Process::CLOCK_MONOTONIC, :float_second)
• Rename monotonic_time → monotonic_millis to make the unit explicit
• Update SlidingWindow to call @clock.monotonic_seconds directly, removing its own conversion helper
• Update RBS signatures for both clock types and SlidingWindow

Acceptance criteria

• SystemClock#monotonic_seconds returns float seconds
• SystemClock#monotonic_millis replaces monotonic_time
• NullClock exposes both methods
• SlidingWindow removes its private conversion helper and calls @clock.monotonic_seconds
• RBS updated throughout
• system_clock_spec covers both new methods