Add error handling for ambiguous bases in k-mer encoding

This commit introduces error handling for ambiguous DNA bases (N, R, Y, W, S, K, M, B, D, H, V) in k-mer encoding. It adds new functions IterNormalizedKmersWithErrors and EncodeNormalizedKmersWithErrors that track and encode the number of ambiguous bases in each k-mer using error markers in the top 2 bits. The commit also updates the version string to reflect the latest changes.

pkg/obikmer/encodefourmer.go

@@ -5,6 +5,10 @@ import (
 	"git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obiutils"
 )
 
+// __single_base_code__ encodes DNA bases to 2-bit values.
+// Standard bases: A=0, C=1, G=2, T/U=3
+// Ambiguous bases (N, R, Y, W, S, K, M, B, D, H, V): 0xFF (255) to signal error
+
 var __single_base_code__ = []byte{0,
 	//     A,  B,  C,  D,
 	0, 0, 1, 0,