Fix primitive default-value round-trip and preserve -0.0 in the codec

[ghostdogpr]

Two encode/decode consistency bugs in the primitive codec, both surfaced by a fresh bug-hunt pass.

1. Non-zero primitive defaults corrupt the round-trip

The deriver populates a SimpleField's decode default from the schema default, which zio-blocks derives from the Scala case-class default parameter. So for case class Foo(x: Int = 5), the field's decode default is 5. But encode omits a primitive field whenever it equals the type's zero (value == 0, value.isEmpty, …) — it never consults the declared default. Result:

encode(Foo(0))  // x == 0 → omitted
decode(...)     // x absent → filled with 5
// => Foo(5), silently corrupting 0 into 5

Encode categorically ignores the declared default for every field category, so decode must too. For primitive fields the decode default is now the proto3 zero (getDefaultValue), matching encode. Message / enum / option / collection / transform defaults are untouched.

2. -0.0 is dropped

Presence for Double/Float used value == 0d / value == 0f. Since -0.0 == 0.0 under IEEE equality, -0.0 was treated as the omittable default and round-tripped to +0.0 — also diverging from protobuf-java, whose generated code gates on doubleToRawLongBits(value) != 0. Presence now checks raw bits, with a value == 0 short-circuit so ordinary non-zero values never call doubleToRawLongBits (keeping the JS hot path unaffected).

Tests

Two regression tests (red before the fix, green after): a primitive field with a non-zero default round-tripping its zero value, and -0.0 double/float fields preserving their sign bit (compared by raw bits). Full core suite: 139 passed.