fix(unstable): correct Parser.Range for non-suffix highlights

Parser.Range used len(data)-len(highlight), which only matches suffix
slices. Single-byte highlights like b[0:1] are subslices of the
remaining buffer, so the wrong offset pointed at the end of the
document (issue #1047). Use pointer-based subslice offset like
wrapDecodeError.

Add regression tests for unstable.Parser and toml.Unmarshal error
positions and human-readable output.

Co-authored-by: Thomas Pelletier <thomas@pelletier.dev>

unstable/parser_range_test.go

@@ -0,0 +1,36 @@
+package unstable
+
+import (
+	"errors"
+	"testing"
+)
+
+// Regression test for https://github.com/pelletier/go-toml/issues/1047:
+// Parser.Range must use the real slice offset, not len(data)-len(slice).
+func TestParser_Range_HighlightAfterComment(t *testing.T) {
+	input := []byte("# comment\n= \"value\"")
+
+	var p Parser
+	p.Reset(input)
+	for p.NextExpression() {
+	}
+	err := p.Error()
+	if err == nil {
+		t.Fatal("expected an error")
+	}
+
+	var perr *ParserError
+	if !errors.As(err, &perr) {
+		t.Fatalf("expected *ParserError, got %T", err)
+	}
+
+	r := p.Range(perr.Highlight)
+	shape := p.Shape(r)
+
+	if r.Offset != 10 {
+		t.Errorf("Range offset: got %d, want 10", r.Offset)
+	}
+	if shape.Start.Line != 2 || shape.Start.Column != 1 {
+		t.Errorf("position: got %d:%d, want 2:1", shape.Start.Line, shape.Start.Column)
+	}
+}