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feat(elreal): Phase E.5 -- inverse trig family with depth-1 derivative refinement (#897)

Phase E.5 of epic #873 (resolves #891). Four inverse trigonometric
functions: asin, acos, atan, atan2. Same uniform pattern as E.3/E.4:
std library at depth 0, derivative-based correction at depth 1.

Per-function derivatives:
  d/dx asin(x)  =  1 / sqrt(1 - x^2)     valid for |x| < 1
  d/dx acos(x)  = -1 / sqrt(1 - x^2)     valid for |x| < 1
  d/dx atan(x)  =  1 / (1 + x^2)         always finite
  d  atan2(y,x): partials
    d/dy =  x / (x^2 + y^2)
    d/dx = -y / (x^2 + y^2)

Edge cases (per IEEE-754 / std lib):
  asin/acos at |x| > 1     -> NaN
  asin/acos at |x| = 1     -> well-defined, derivative diverges; depth-1
                              skipped
  atan(+/-inf)             -> +/-pi/2 (well-defined; depth-1 = 0 since
                              the derivative goes to 0)
  atan2(0, 0)              -> 0 (C convention; depth-1 skipped)

Note: this phase does NOT require pi-based range reduction. The std lib
handles all the work at depth 0, and the derivatives are closed-form
rational expressions in the argument(s). Range reduction is the hard
problem in forward trig (Phase E.6), not inverse trig.

Tests under elastic/elreal/math/inverse_trig.cpp:
  - Anchor values (atan(0)=0, atan(1)=pi/4, asin(+/-1)=+/-pi/2,
    acos(-1)=pi)
  - atan2 four-quadrant: (0,1)=0, (1,0)=pi/2, (0,-1)=pi, (-1,0)=-pi/2,
    (1,1)=pi/4, (1,-1)=3pi/4
  - Cross-validation vs std::asin/acos/atan
  - asin/acos at |x|>1 = NaN
  - atan(+/-inf) = +/-pi/2
  - asin(x) + acos(x) == pi/2 identity
  - atan2(y, x>0) == atan(y/x) consistency
  - NaN propagation through all four
  - Depth-1 propagation on rational input

21 elreal binaries (1 new + 20 prior) PASS on gcc 13 and clang.

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

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