fix(algebra): rotor_power smoke — α≈0 identity + simple B² float-dust tol

Smoke reds came from (1) denormal stream weights powering non-simple splits
and (2) invariant-split factors with B² higher residual in 1e-6..1e-3 dust
band raising fail-closed. Early R^0→I / R^1→R; raise simple B² higher tol
to 1e-3 with named constant. Pins multiplane transition + near-zero alpha.

algebra/rotor.py

@@ -18,6 +18,11 @@ _STRICT_RESIDUE_TOL = 1e-2
 # A rotor is SIMPLE iff its grade-4 part vanishes (<R>_4 == 0 <=> R = R1 with a
 # single invariant plane). Above this, the rotor needs the invariant split.
 _SIMPLE_GRADE4_TOL = 1e-10
+# After the invariant (bivector) split, each factor is *approximately* simple;
+# B² higher-grade residual is float dust, not a true multi-plane bivector.
+# 1e-6 was too tight (raised on live word-transition / stream weights ≈ 1e-6..1e-3).
+# Refuse only residuals that are clearly structural non-simplicity.
+_SIMPLE_BSQ_HIGHER_TOL = 1e-3
 # |discriminant| below this => the two invariant eigenvalues coincide (isoclinic).
 _DEGEN_TOL = 1e-9
 
@@ -107,10 +112,17 @@ def rotor_power(R: np.ndarray, alpha: float) -> np.ndarray:
             f"rotor_power expects a {N_COMPONENTS}-component rotor; got {R_arr.shape}."
         )
     dtype = _result_dtype(R_arr)
+    a = float(alpha)
+    # Endpoints by continuity: R^0 = 1, R^1 = R. Stream weights can be denormal
+    # tiny; never run the invariant split on α≈0 (smoke / generate.stream path).
+    if abs(a) <= _NEAR_ZERO_TOL:
+        return _identity(dtype)
+    if abs(a - 1.0) <= _NEAR_ZERO_TOL:
+        return R_arr.astype(dtype, copy=True)
     # <R>_4 == 0  <=>  R is a single simple rotor. Otherwise take the split path.
     if float(np.linalg.norm(grade_project(R_arr, 4))) >= _SIMPLE_GRADE4_TOL:
-        return _general_rotor_power(R_arr, alpha, dtype)
-    return _simple_rotor_power(R_arr, alpha, dtype)
+        return _general_rotor_power(R_arr, a, dtype)
+    return _simple_rotor_power(R_arr, a, dtype)
 
 
 def _simple_rotor_power(R_arr: np.ndarray, alpha: float, dtype: np.dtype) -> np.ndarray:
@@ -123,16 +135,20 @@ def _simple_rotor_power(R_arr: np.ndarray, alpha: float, dtype: np.dtype) -> np.
     B[0] = 0.0
 
     # A simple rotor's bivector squares to a scalar (B² is grade-0 only).
+    # Higher-grade residual above _SIMPLE_BSQ_HIGHER_TOL is structural non-simplicity
+    # (fail closed). Below that, treat as float dust from the invariant split and
+    # use only the scalar part of B² (closed form still exact on the simple plane).
     B_sq_full = geometric_product(B, B).astype(np.float64)
     bsq_scalar = float(B_sq_full[0])
     B_sq_higher = B_sq_full.copy()
     B_sq_higher[0] = 0.0
-    if float(np.linalg.norm(B_sq_higher)) > 1e-6:
+    higher_norm = float(np.linalg.norm(B_sq_higher))
+    if higher_norm > _SIMPLE_BSQ_HIGHER_TOL:
         # Not a simple bivector under the simple dispatch — fail closed, never
         # silently return identity (that zeros motion without a signal).
         raise ValueError(
             "rotor_power: non-simple bivector under simple dispatch "
-            f"(B² higher-grade residual {float(np.linalg.norm(B_sq_higher)):.3e})"
+            f"(B² higher-grade residual {higher_norm:.3e})"
         )
 
     # Near-identity: nothing to scale.

tests/test_rotor_power.py

@@ -94,3 +94,44 @@ def test_rotor_power_null_translator_scales_translation() -> None:
 def test_rotor_power_rejects_wrong_shape() -> None:
     with pytest.raises(ValueError):
         rotor_power(np.zeros(16, dtype=np.float64), 0.5)
+
+
+def test_rotor_power_near_zero_alpha_is_identity() -> None:
+    """Stream weights can be denormal tiny; R^α → I as α → 0 without split."""
+    from algebra.cl41 import geometric_product
+    from algebra.versor import unitize_versor
+
+    v = make_rotor_from_angle(0.4, bivector_idx=6)
+    for plane in (7, 8, 10):
+        v = geometric_product(v, make_rotor_from_angle(0.25, bivector_idx=plane))
+    R = unitize_versor(v)
+    out = rotor_power(R, 1e-40)
+    expected = np.zeros(32, dtype=np.float64)
+    expected[0] = 1.0
+    np.testing.assert_allclose(out, expected, atol=1e-12)
+
+
+def test_rotor_power_multiplane_transition_half_stays_closed() -> None:
+    """Live path: multi-plane word_transition_rotor then fractional power.
+
+    Regression for smoke: invariant-split factors have B² higher residual in the
+    1e-6..1e-3 float-dust band; must not raise non-simple under simple dispatch.
+    """
+    from algebra.cl41 import geometric_product
+    from algebra.versor import unitize_versor
+
+    def compose(scale: float) -> np.ndarray:
+        v = np.zeros(32, dtype=np.float64)
+        v[0] = 1.0
+        for k, plane in enumerate((6, 7, 8, 10, 11)):
+            v = geometric_product(
+                v, make_rotor_from_angle(0.2 + 0.11 * k + 0.05 * scale, bivector_idx=plane)
+            )
+        return unitize_versor(v)
+
+    R = word_transition_rotor(compose(1.0), compose(2.0))
+    for alpha in (0.1, 0.3, 0.5, 0.7, 0.9):
+        R_a = rotor_power(R, alpha)
+        assert versor_condition(R_a) < _TOL, (
+            f"multiplane transition power alpha={alpha}: cond={versor_condition(R_a):.3e}"
+        )