onnx/special: add MeanVarianceNormalization, Hardmax, and hyperbolic ops

[axiom-of-choice]

Three ONNX ops composed from existing graph ops, no new kernels:

• MeanVarianceNormalization (onnx: MeanVarianceNormalization #123): (x - mean) / sqrt(var + 1e-9) over axes (default [0,2,3]), var = mean(x*x) - mean(x)^2, fixed 1e-9 epsilon.
• Hardmax (onnx: Hardmax #122): one-hot of the argmax (lowest index on ties) via argmax + select against a baked arange; 2D [C, W] only with axis in {-2,-1,0,1}, raises on higher rank (matches the ANE ArgMax constraint).
• Sinh/Cosh/Asinh/Acosh/Atanh (onnx: Sinh, Cosh, and the inverse hyperbolics #121): added to aneforge.special (decompositions in exp/log/sqrt) and routed from ONNX; fp16 overflow near |x| ~ 11 documented, tests use the finite range.

Testing

• Build-level and numeric tests in tests/test_onnx.py (oracle onnxruntime) plus special-function tests in tests/test_special_trig.py (oracle numpy).
• On-device M2 Pro: tests/test_special_trig.py tests/test_onnx.py → 200 passed. Errors: MVN 2.2e-3 abs; Hardmax exact (0.0); hyperbolics ≤3.4e-3 rel.
• Off-device suite: 407 passed, 2 skipped.
• ruff, pylint 2-space gate, pyright aneforge, compileall clean.

[sbryngelson]

Hardmax is solid -- checking the tie cases against onnxruntime was the right call, and it passes on
device. The other two parts need changes.

MeanVarianceNormalization computes variance one-pass as E[x^2] - E[x]^2, which cancels in fp16 as
soon as the mean is large relative to the spread. Measured against the two-pass form on the same
graph, same input, [1,3,4,4] standard normal shifted by a constant:

mean shift    one-pass (this PR)    two-pass
       0.0                0.0018      0.0010
       5.0                0.0463      0.0052
      50.0                   inf      0.0234
     200.0                   inf      0.2921

The test only uses standard_normal, which is the one case where the two agree. Real MVN inputs are
not centered -- that's what the op is for.

The inf is a second, independent bug: var.adds(1e-9) is not a guard, because the ANE flushes
subnormals. Everything below fp16's smallest normal comes back as exactly zero:

eps      fp16 value    sqrt(0 + eps) on ANE
1e-09     0.000e+00                     0.0
1e-07     1.192e-07                     0.0      <- representable as a subnormal, still flushed
1e-06     1.013e-06                     0.0
6e-05     6.002e-05                7.8125e-03
1e-04     1.000e-04                1.0338e-02

So the epsilon has to be at least ~6e-5 to exist at all. Two-pass makes var >= 0 by construction, so
it only matters for genuinely constant input, but it should still be a real number.

On the hyperbolic ops: they pass, but the test metric can't see what it's measuring. abs(out - ref).max() / abs(ref).max()
over linspace(-10, 10) divides by sinh(10) = 11013, so the 5e-3 tolerance permits 55 in absolute
error. Per-point relative error:

sinh(0.001)    2.4% off
atanh(0.003)   4.1% off
asinh(-10)     2.1% off

Only asinh has a clean fix, and I've applied it: asinh is odd, so evaluating on |x| and signing
back avoids the cancellation in log(x + sqrt(x^2+1)) for x << 0. That takes x=-10 from 2.1% to 0.0%
with no regression on positive x.

For sinh and atanh I did not change the formula. expm1 and log1p fix the origin exactly but
are only valid on |x| <= ~0.7 and argument in [-0.5, 1], so using them would need a domain split like
exp_wide/log_wide already do. Instead I documented the actual limit, since the current docstrings
imply overflow near 11 is the only constraint. Happy to take the domain split instead if you'd
rather have the accuracy -- say the word.

Note the patch also updates test_hyperbolic_ops_build, since asinh's terminal op becomes mul.

Verified on device: 203 passed across test_special_trig.py and test_onnx.py, ruff clean. Reverting
just the source changes makes the new tests fail (MVN non-finite at shift 50, asinh 2.08% at x=-10),
so they do pin the behaviour.

diff --git a/aneforge/onnx.py b/aneforge/onnx.py
index be89496..e855994 100644
--- a/aneforge/onnx.py
+++ b/aneforge/onnx.py
@@ -895,8 +895,9 @@ def _mvn(node, ins, a, i):
     return (x - mean) / np.sqrt(var + 1e-9)
   axes = tuple(int(v) % len(x.shape) for v in a.get("axes", [0, 2, 3]))
   mean = x.mean(axes)
-  var = (x * x).mean(axes) - mean * mean
-  return (x - mean) / var.adds(1e-9).sqrt()
+  d = x - mean
+  var = (d * d).mean(axes)                   # two-pass: E[x^2]-E[x]^2 cancels to <=0 in fp16 once mean >> std
+  return d / var.adds(1e-4).sqrt()           # eps below fp16's min normal (6.1e-5) flushes to zero on the ANE
 @onnx_op("Hardmax")
 def _hardmax(node, ins, a, i):
   """Hardmax: one-hot of the argmax (lowest index on ties); 2D [C,W] only, axis in {-2,-1,0,1}."""
diff --git a/aneforge/special.py b/aneforge/special.py
index 375fdf2..4713065 100644
--- a/aneforge/special.py
+++ b/aneforge/special.py
@@ -221,7 +221,11 @@ def log_wide(x: Tensor, sqrts: int = 3) -> Tensor:
 # hyperbolic trig
 
 def sinh(x: Tensor) -> Tensor:
-  """sinh(x) for |x| <= ~10; overflows fp16 near ln(65504) ~ 11 (same wall as softplus)."""
+  """sinh(x) for |x| <= ~10; overflows fp16 near ln(65504) ~ 11 (same wall as softplus).
+
+  Accurate in absolute terms, not relative: e^x - e^-x cancels near the origin, so the relative
+  error reaches ~2.4% for |x| < 0.01 (absolute error stays ~2e-5). Use expm1 if you need the
+  small-argument regime; its Taylor form is only valid for |x| <= ~0.7."""
   return (x.exp() - (x * -1.0).exp()) * 0.5
 
 
@@ -231,8 +235,11 @@ def cosh(x: Tensor) -> Tensor:
 
 
 def asinh(x: Tensor) -> Tensor:
-  """asinh(x) for |x| <= ~10; domain is all real x, but fp16 overflows for large |x|."""
-  return (x + (x * x).adds(1.0).sqrt()).log()
+  """asinh(x) for |x| <= ~10; domain is all real x, but fp16 overflows for large |x|.
+
+  Evaluated on |x| and signed back: asinh is odd, and log(x + sqrt(x^2+1)) cancels for x << 0
+  (2.1% relative error at x=-10 in fp16, vs 0.0% for this form)."""
+  return (x.abs() + (x * x).adds(1.0).sqrt()).log() * x.sign()
 
 
 def acosh(x: Tensor) -> Tensor:
@@ -241,7 +248,11 @@ def acosh(x: Tensor) -> Tensor:
 
 
 def atanh(x: Tensor) -> Tensor:
-  """atanh(x) for |x| < 1; singular at |x| == 1."""
+  """atanh(x) for |x| < 1; singular at |x| == 1.
+
+  Accurate in absolute terms, not relative: (1+x)/(1-x) rounds to ~1 near the origin, so the
+  relative error reaches ~4% for |x| < 0.01 (absolute error stays ~1e-4). log1p fixes the
+  small-argument regime but is only valid for its argument in [-0.5, 1], i.e. |x| <= 1/3."""
   return ((x.adds(1.0)) / (x * -1.0).adds(1.0)).log() * 0.5
 
 
diff --git a/tests/test_onnx.py b/tests/test_onnx.py
index 83d6dc8..a88d485 100644
--- a/tests/test_onnx.py
+++ b/tests/test_onnx.py
@@ -1470,7 +1470,7 @@ def test_negative_axis_matches_positive_equivalent():
 # -- hyperbolic ops, MeanVarianceNormalization, Hardmax --------------------- #
 
 def test_hyperbolic_ops_build():
-  for op, want in [("Sinh", "muls"), ("Cosh", "muls"), ("Asinh", "log"), ("Acosh", "log"), ("Atanh", "muls")]:
+  for op, want in [("Sinh", "muls"), ("Cosh", "muls"), ("Asinh", "mul"), ("Acosh", "log"), ("Atanh", "muls")]:
     m = _model([helper.make_node(op, ["x"], ["y"])], [_vi("x", [1, 4])], [_vi("y", [1, 4])])
     _, out = af.onnx_to_tensor(m); assert out.shape == (1, 4) and out.op == want, f"{op} -> {out.op}"
 
@@ -1487,12 +1487,15 @@ def test_hardmax_build():
   with pytest.raises(NotImplementedError): af.onnx_to_tensor(m)
 
 @requires_ane
-def test_mvn_numeric():
+@pytest.mark.parametrize("shift", [0.0, 5.0, 50.0])
+def test_mvn_numeric(shift):
+  """Shifted means are the fp16 trap: one-pass E[x^2]-E[x]^2 cancels and divides by zero at shift=50."""
   pytest.importorskip("onnxruntime")
-  rng = np.random.default_rng(40); x = rng.standard_normal((1, 3, 4, 4)).astype(np.float32)
+  rng = np.random.default_rng(40); x = (rng.standard_normal((1, 3, 4, 4)) + shift).astype(np.float32)
   m = _model([helper.make_node("MeanVarianceNormalization", ["x"], ["y"])], [_vi("x", [1, 3, 4, 4])], [_vi("y", [1, 3, 4, 4])])
   got, ref = _run_vs_ort(m, x)
-  assert np.abs(got - ref).max() < 1e-2
+  assert np.isfinite(got).all(), f"MVN produced non-finite output at mean shift {shift}"
+  assert np.abs(got - ref).max() < 5e-2
 
 @requires_ane
 def test_hardmax_numeric():
diff --git a/tests/test_special_trig.py b/tests/test_special_trig.py
index 9b138bf..2c7b293 100644
--- a/tests/test_special_trig.py
+++ b/tests/test_special_trig.py
@@ -106,6 +106,17 @@ def test_asinh_decomposition_matches_numpy():
   assert np.abs(out - ref).max() < 1e-1
 
 
+def test_asinh_is_odd_and_accurate_for_negative_x():
+  """Per-point relative error, which an error-over-max-|ref| metric cannot see: the naive
+  log(x + sqrt(x^2+1)) cancels for x << 0 and is 2.1% off at x=-10."""
+  x = np.array([[-10.0, -6.0, -3.0, -1.0, 1.0, 3.0, 6.0, 10.0]], np.float16)
+  out = _run(special.asinh, x)
+  ref = np.arcsinh(x.astype(np.float32))
+  rel = np.abs(out - ref) / np.abs(ref)
+  assert rel.max() < 5e-3, f"asinh per-point relerr {rel.max():.2%} at x={float(x[0][rel.argmax()])}"
+  assert np.abs(out + out[:, ::-1]).max() < 1e-3, "asinh must be odd"
+
+
 def test_acosh_decomposition_matches_numpy():
   x = np.linspace(1.0001, 10.0, 128).astype(np.float16).reshape(1, 128)
   out = _run(special.acosh, x)