stillwater-sc / universal / 24948080228

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feat(math): add constexpr log (natural logarithm) to sw::math::constexpr_math (#771)

* feat(math): add constexpr log (natural logarithm) to sw::math::constexpr_math

Adds constexpr<float|double> log to the constexpr_math facility (#763
Epic, #423/PR #762 substrate). Tier-2 sub-issue: natural log used by
dbns and useful for general literal construction.

Implementation: log(x) = log2(x) * ln(2) -- thin wrapper over the
existing log2 machinery. Special cases (NaN, x<0, 0, +inf) are handled
explicitly before the multiply rather than relying on log2 to propagate
them; clang's stricter constexpr evaluator rejects "arithmetic produces
a NaN" so log2(NaN) * LN2 would trip the diagnostic even though log2
itself returned NaN cleanly.

Verification:
  - 9 static_asserts: log(1) == 0, log(2) ~= ln(2) (LN2 anchor),
    log(e) ~= 1, log(10) ~= ln(10), log(8) == 3*log(2) cross-check,
    plus NaN / +/-inf / negative-input special values
  - Runtime sweep over 22 hand-picked points spanning 1e-300 to 1e300
  - Float sweep over 10 points

Out-of-band accuracy stress (100K random samples in [1e-100, 1e100]):
  - max relative error 1.29e-16 = 0.58 x double epsilon (sub-ulp)
  - tighter than the standalone log2 because the LN2 multiply often
    rounds favorably for non-pow-2 inputs

cm_log, cm_log2, cm_exp2 all PASS on gcc and clang in build_ci /
build_ci_clang.

Resolves #766
Relates to #763 (Epic)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* test(math): pin log(-0) and use tolerance for log cross-check (CodeRabbit)

Two Nitpicks from CodeRabbit on PR #771:

1. Pin -0 behavior explicitly: cm::log(-0.0) and cm::log(-0.0f) must
   return -infinity (the IEEE-754 zero branch matches both signed
   zeros). The implementation already handled this correctly via
   x == 0.0 (which compares equal for both +0 and -0), but a regression
   test prevents accidental breakage.

2. Replace exact-equality cross-check (log(8) == 3*log(2)) with a
   toleranc... (continued)

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