Reorganize math package into subpackages

- Create math/big for big.Int utilities (HexOrDecimal256, U256, parsing)
- Create math/safe for overflow-safe arithmetic (SafeAdd, SafeMul, etc.)
- Move averager files to root package level
- Add re-exports in root package for backwards compatibility
- Update README with new package structure documentation
- Remove old math/meter directory (consolidated elsewhere)
- Remove redundant math/ subdirectory structure

go.mod

@@ -2,4 +2,10 @@ module github.com/luxfi/math
 
 go 1.25.5
 
-require github.com/luxfi/sampler v1.0.0
+require (
+	github.com/luxfi/math/big v1.0.0
+	github.com/luxfi/sampler v1.0.0
+	golang.org/x/exp v0.0.0-20241217172543-b2144cdd0a67
+)
+
+replace github.com/luxfi/math/big => ./big