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fix(posit): fix division bugs and complete regression tests (#534) (#537)

* fix(posit): fix division bugs and complete regression tests (#534)

Two bugs in posit2's division pipeline:

1. Missing setradix() in normalizeDivision's multi-limb path caused
   blocksignificand::div() to use wrong radix (bfbits vs divbits),
   placing quotient bits 2 positions too high (e.g. 10/2=4).

2. Missing sticky bits in convert() — only nbits+4 fraction bits were
   extracted from the blocktriple, silently discarding remaining quotient
   bits needed for correct round-to-even decisions (-1 ULP errors).

Also completes the posit2 division regression test suite to match posit1:
- Random tests at levels 2-4 for posit<16,2> through posit<64,4>
- Adversarial worst-case division test functions
- Guards posit1-specific functions in posit_test_suite_randoms.hpp so
  both posit and posit1 can share the same test infrastructure.

Verified with both gcc and clang. No regressions in add/sub/mul.

* code hygiene

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

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