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/go.mod

@@ -1,3 +1,5 @@
 module fuj-management/go
 
 go 1.26.1
+
+require golang.org/x/text v0.36.0