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feat(elreal): atan / asin / acos (Phase 7.5, #931) (#1043)

Inverse trig in math/trigonometry.hpp, on the 7.1 (pi) / 7.2 (sqrt) foundation:
- atan(x) = 2*atan(x/(1+sqrt(1+x^2))) -- argument halving until small, then the
  alternating Taylor series, scaled back by 2^k.
- asin(x): atan(x/sqrt(1-x^2)) for |x|<=1/sqrt(2); else the reflection
  asin(x)=sign(x)*(pi/2 - asin(sqrt(1-x^2))) so the atan argument stays bounded
  (a direct x/sqrt(1-x^2) blows up near |x|=1 and underflows the host range).
  |x|=1 -> +/-pi/2; |x|>1 -> empty.
- acos(x) = pi/2 - asin(x).

Host scope: atan is robust and tested on {double, float}. asin/acos are the
deepest compositions in the suite (sqrt . atan . div . pi); a float host's narrow
exponent range underflows the intermediate expansions toward the domain boundary
(where 1-x^2 also cancels), so they are tested on {double}. Documented in the
header.

Tests: value vs std::<fn>, asin(x)+acos(x)==pi/2, atan(x)+atan(1/x)==pi/2,
4*atan(1)==pi, parity, domain; 0-overlap on every result.

All elreal tests pass on gcc and clang (Release) and with assertions on (~1s);
RISC-V cross-compiles clean; ASCII clean.

Relates to #931 (sub-phase 7.5).

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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