stillwater-sc / universal / 29643890345

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feat(efloat): implement math/trigonometry.hpp (sin, cos, tan, asin, acos, atan, atan2) (#1137)

* feat(efloat): implement math/trigonometry.hpp (sin, cos, tan, asin, acos, atan, atan2)

Adds the trigonometry header for the arbitrary-precision efloat template,
completing the last missing piece of the efloat mathematical library (parent
#1092). Native arbitrary-precision implementations, not double-delegating shims:

- sin/cos: argument reduction to [-pi, pi] via r = x - round(x/2pi)*2pi, then
  Taylor series with the precision-aware term.scale()/get_precision() terminator
  used by expm1/log1p.
- tan = sin/cos, with a signed-infinity result at the pole (cos == 0).
- atan: |x|==1 -> pi/4; |x|>1 -> pi/2 - atan(1/|x|); 0.5<|x|<=1 -> addition
  formula atan(1/2) + atan((|x|-1/2)/(1+|x|/2)); else Taylor. Keeps the reduced
  argument small so the series converges quickly (no slow Leibniz tail near 1).
- asin: domain |x|>1 -> NaN; |x|>0.8 -> pi/2 - asin(sqrt(1-x^2)); else Taylor.
- acos = pi/2 - asin.
- atan2: full-quadrant, using the efloat pi constants (NOT bare double literals,
  avoiding the double-contamination bug present in ereal's atan2).

High-precision pi seeded via parse() of a ~62-digit literal (same idiom exp/log
use for ln2); pi/2 and pi/4 derived by exact multiplication. Constants are
capped at ~62 digits like the existing ln2/ln10 -- adequate for the standard
efloat configs; a longer literal or native pi computation can lift that later.

Note: the free abs()/fabs() function is a pre-existing stub that returns its
argument unchanged, so this code takes absolute values via the setsign(false)
idiom the other efloat math headers already use.

Wired into mathlib.hpp after truncate.hpp (trig needs round()). New regression
test elastic/efloat/math/trigonometry.cpp (auto-globbed as target
efloat_trigonometry) covers values, identities (sin^2+cos^2==1), argument
reduction, inverse round-trips, all four atan2 quadrants, domain errors, and
NaN/Inf specia... (continued)

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14 existing lines in 3 files now uncovered.

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