feat(determin): add Dfp::from_signed, DFP_CANONICAL_NAN, DQA CANONICAL_ZERO

Add missing API surface identified in DFP (RFC-0104) and DQA (RFC-0105)
code reviews:

- Dfp::from_signed(mantissa, exponent): creates DFP from signed mantissa,
  extracting sign automatically (RFC-0104 D2)
- DFP_CANONICAL_NAN constant: canonical NaN for pattern-matching (RFC-0104 D3)
- DQA CANONICAL_ZERO constant: canonical zero value=0, scale=0 (RFC-0105 D1)
- Re-export CANONICAL_ZERO from crate root

Author: mmacedoeu

determin/src/dqa.rs

@@ -109,6 +109,9 @@ pub struct Dqa {
     pub scale: u8,
 }
 
+/// Canonical zero value (value=0, scale=0)
+pub const CANONICAL_ZERO: Dqa = Dqa { value: 0, scale: 0 };
+
 impl Dqa {
     /// Create DQA from value and scale
     /// Returns Error::InvalidScale if scale > 18

determin/src/lib.rs

@@ -60,7 +60,7 @@ pub use decimal::{
     MAX_DECIMAL_OP_COST, MAX_DECIMAL_SCALE, MIN_DECIMAL_MANTISSA,
 };
 pub use dmat::{DMat, DmatError, NumericScalar};
-pub use dqa::{dqa_abs, dqa_assign_to_column, dqa_cmp, dqa_negate, Dqa, DqaEncoding, DqaError};
+pub use dqa::{dqa_abs, dqa_assign_to_column, dqa_cmp, dqa_negate, CANONICAL_ZERO, Dqa, DqaEncoding, DqaError};
 pub use dvec::{
     dot_product, norm, normalize, squared_distance, vec_add, vec_mul, vec_scale, vec_sub, DVec,
     DvecError, DvecScalar,
@@ -189,6 +189,23 @@ impl Dfp {
         debug_assert!(self.mantissa % 2 == 1 || self.mantissa == 0);
     }
 
+    /// Create DFP from signed mantissa and exponent
+    /// Extracts sign from the mantissa automatically
+    pub fn from_signed(mantissa: i128, exponent: i32) -> Self {
+        if mantissa == 0 {
+            return Dfp::zero();
+        }
+        let sign = mantissa < 0;
+        let mut dfp = Dfp {
+            mantissa: mantissa.unsigned_abs(),
+            exponent,
+            class: DfpClass::Normal,
+            sign,
+        };
+        dfp.normalize();
+        dfp
+    }
+
     /// Create DFP from i64 (integer)
     pub fn from_i64(val: i64) -> Self {
         if val == 0 {
@@ -350,6 +367,14 @@ pub const DFP_MIN: Dfp = Dfp {
     exponent: DFP_MAX_EXPONENT,
 };
 
+/// Canonical NaN value
+pub const DFP_CANONICAL_NAN: Dfp = Dfp {
+    class: DfpClass::NaN,
+    sign: false,
+    mantissa: 0,
+    exponent: 0,
+};
+
 /// DFP encoding for serialization (24 bytes)
 /// Layout: [mantissa: 16 bytes, exponent: 4 bytes, class_sign: 4 bytes]
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]