fix: support negative precision in Truncate

[Yanhu007]

Fixes #406

Problem

Truncate ignores negative precision values due to a precision >= 0 guard:

d := decimal.RequireFromString("5432")
d.Truncate(-2).String() // returns "5432" (unchanged)
// expected: "5400"

Fix

Remove the precision >= 0 check. The existing rescale logic already handles negative precision correctly — it truncates integer digits.

decimal.RequireFromString("5432").Truncate(-2)  // "5400" ✅
decimal.RequireFromString("5432").Truncate(-1)  // "5430" ✅
decimal.RequireFromString("123.456").Truncate(2) // "123.45" ✅ (unchanged)

Testing

All existing tests pass. The fix is a single-line change (removing precision >= 0 &&).