feat(go/M2.1): port czech.Normalize — NFKD + Mn strip + lowercase

Adds internal/domain/czech.Normalize, the first pure-domain function in
the Go rewrite (M2 milestone). Matches Python czech_utils.normalize byte-
for-byte: NFKD decompose via golang.org/x/text/unicode/norm, drop Mn-
category combining marks (unicode.Mn, not IsMark, to match Python's
unicodedata.combining() semantics), then strings.ToLower.

Includes 13-case table-driven test; all inputs spot-checked against the
Python implementation before locking. Adds golang.org/x/text v0.36.0 as
first external dependency.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

go/internal/domain/czech/normalize_test.go

@@ -0,0 +1,31 @@
+package czech
+
+import "testing"
+
+func TestNormalize(t *testing.T) {
+	cases := []struct {
+		in   string
+		want string
+	}{
+		{"Honza", "honza"},
+		{"žluťoučký", "zlutoucky"},
+		{"Příliš", "prilis"},
+		{"Dvořák", "dvorak"},
+		{"Růžena", "ruzena"},
+		{"Čeněk", "cenek"},
+		{"Kačer", "kacer"},
+		{"", ""},
+		{"prilis", "prilis"},                     // idempotent
+		{"Jan Novák", "jan novak"},               // whitespace preserved
+		{"é", "e"},                               // precomposed é (NFC)
+		{"é", "e"},                              // decomposed e + combining acute
+		{"Ondřej Procházka", "ondrej prochazka"}, // realistic full name
+	}
+
+	for _, tc := range cases {
+		got := Normalize(tc.in)
+		if got != tc.want {
+			t.Errorf("Normalize(%q) = %q, want %q", tc.in, got, tc.want)
+		}
+	}
+}