Use pointer sized words on all platforms

[fingolfin]

Word was unsigned long, meant as the native word size. On Windows that has 32 bits, so vectors used half-width words, and their external representation, a raw copy of the words, no longer matched files written elsewhere, such as orb's test data. uintptr_t keeps the intent on every platform; the constants follow.

The address of the gf2 arena was rounded through unsigned long too, truncating it wherever the module loads above 4 GB, as 64-bit DLLs do.

Assisted-by: Claude Code (Opus 5)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.03%. Comparing base (6fac179) to head (337762b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #56   +/-   ##
=======================================
  Coverage   61.03%   61.03%           
=======================================
  Files           9        9           
  Lines        5731     5731           
=======================================
  Hits         3498     3498           
  Misses       2233     2233           

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