ffi 0.4: focused FP-aggregate (HFA) baseline — FQuad + DQuad

91/91 regression tests pass (+ffi-04-fp-struct).

Single-file regression net for the all-float / all-double aggregate
ABI path:

  FQuad — 16 B, 4×f32   (same slot as ffi-02's Vec4f)
  DQuad — 32 B, 4×f64   (UIEdgeInsets-shape — the f32-vs-f64 landmine)

Already nominally covered by ffi-02's Vec4f, but pinning it as a
focused single-file test means a future ABI rule change that breaks
the HFA path fails *this* test directly without a noisy drag-in from
the multi-shape baseline.

DQuad at 32 B straddles the AAPCS64 HFA limit (≤4 floats of same
type, total ≤64 B); it stays as a struct value passed through
v0..v3 rather than going indirect. The snapshot confirms the values
arrive intact.

tests/expected/ffi-04-fp-struct.exit

@@ -0,0 +1 @@
+0

tests/expected/ffi-04-fp-struct.txt

@@ -0,0 +1,6 @@
+fquad make = (1.000000, 2.000000, 3.000000, 4.000000)
+fquad rev  = (4.000000, 3.000000, 2.000000, 1.000000)
+fquad sum  = 10.000000
+dquad make = (1.500000, 2.500000, 3.500000, 4.500000)
+dquad rev  = (4.500000, 3.500000, 2.500000, 1.500000)
+dquad sum  = 12.000000