Introduce new sequence matching logic and corpus handling

This commit introduces the core matching functionality, including parallel corpus loading, the `Corpus` data structure, and implementations for `matchNaive` and `matchApat`. It also defines new matching rules, variant handling, and comprehensive unit and integration tests to validate the new logic and parameter handling.

pkg/obiprimer/match_naive.go

@@ -0,0 +1,78 @@
+package obiprimer
+
+import "git.metabarcoding.org/obitools/obitools4/obitools4/pkg/obiseq"
+
+// baseCode maps a nucleotide to 0..3 (A,C,G,T; U is T) and everything else to 4,
+// which never equals a word base.
+var baseCode = func() (t [256]byte) {
+	for i := range t {
+		t[i] = 4
+	}
+	for i, c := range "ACGT" {
+		t[c], t[c+'a'-'A'] = byte(i), byte(i)
+	}
+	t['U'], t['u'] = 3, 3
+	return
+}()
+
+func codesOf(s string) []byte {
+	out := make([]byte, len(s))
+	for i := range out {
+		out[i] = baseCode[s[i]]
+	}
+	return out
+}
+
+// matchNaive is the reference implementation: every window of every sequence
+// is compared with every word, base by base. It is slow and exists to define
+// the exact semantics (see MatchParams) that faster implementations must
+// reproduce.
+func matchNaive(c *Corpus, words []uint64, p MatchParams) ([]WordMatches, error) {
+	if err := p.validate(); err != nil {
+		return nil, err
+	}
+	fwd, rev := wordStrings(words, p.K)
+	fc, rc := make([][]byte, len(words)), make([][]byte, len(words))
+	for i := range words {
+		fc[i], rc[i] = codesOf(fwd[i]), codesOf(rev[i])
+	}
+
+	mk := func() scanFunc {
+		var codes []byte
+		return func(seq *obiseq.BioSequence, emit func(word, pos, errors int, reverse bool)) {
+			raw := seq.Sequence()
+			n := len(raw)
+			if n < p.K {
+				return
+			}
+			codes = codes[:0]
+			for _, b := range raw {
+				codes = append(codes, baseCode[b])
+			}
+			windows := n - p.K + 1
+			if p.Circular {
+				codes = append(codes, codes[:p.K-1]...)
+				windows = n
+			}
+			for pos := range windows {
+				win := codes[pos : pos+p.K]
+				for w := range words {
+					for strand, pat := range [2][]byte{fc[w], rc[w]} {
+						errs := 0
+						for j, b := range pat {
+							if win[j] != b {
+								if errs++; errs > p.MaxErrors {
+									break
+								}
+							}
+						}
+						if errs <= p.MaxErrors {
+							emit(w, pos, errs, strand == 1)
+						}
+					}
+				}
+			}
+		}
+	}
+	return matchCorpus(c, words, mk), nil
+}