Remove cap tricks, use address comparison for subslice offset

Replace cap(parent) - cap(subslice) with a straightforward scan
that compares element addresses: &data[i] == &subslice[0]. This is
well-defined Go pointer comparison on elements of the same backing
array, with no dependency on capacity semantics, reflect, or unsafe.

The scan is O(n) but only runs on error paths, and TOML documents
are small per the project's design constraints.

Also remove the Offset field from ParserError and the setErrOffset
machinery — the offset is computed at the point of consumption
(wrapDecodeError, Parser.Range) rather than cached on the error.

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

unstable/parser_test.go

@@ -766,50 +766,6 @@ func TestErrorHighlightPositions(t *testing.T) {
 	}
 }
 
-func TestParserError_Offset(t *testing.T) {
-	examples := []struct {
-		desc       string
-		input      string
-		wantOffset int
-	}{
-		{
-			desc:       "error after comment",
-			input:      "# comment\n= \"value\"",
-			wantOffset: 10,
-		},
-		{
-			desc:       "error on first line",
-			input:      "= \"value\"",
-			wantOffset: 0,
-		},
-		{
-			desc:       "error after two lines",
-			input:      "a = 1\n= \"value\"",
-			wantOffset: 6,
-		},
-	}
-
-	for _, e := range examples {
-		t.Run(e.desc, func(t *testing.T) {
-			p := Parser{}
-			p.Reset([]byte(e.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)
-			}
-			if perr.Offset != e.wantOffset {
-				t.Errorf("offset: got %d, want %d", perr.Offset, e.wantOffset)
-			}
-		})
-	}
-}
-
 func ExampleParser() {
 	doc := `
 	hello = "world"