Address review: TableRelation validation carve-out, RecordRef nits

- tablerelation: fields with ValidateTableRelation/TestTableRelation = false
  are skipped by codeunit 134926 and may be longer (filter/totaling
  fields); shorter is still a finding. Add Code/Text type rule, BaseApp
  evidence, a ValidateTableRelation = false filter field to the good
  fixture, and the carve-out to the data-modeling cue.
- recordref: add FindLast/Next, relabel SplitLocalTableFilter as filter
  splitting, mark fixtures as test-library-style helpers.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

microsoft/knowledge/data-modeling/tablerelation-field-length-must-match-related-field.bad.al

@@ -16,8 +16,8 @@ table 50102 "Sample Contract"
     {
         field(1; "Code"; Code[20]) { }
         // Code[10] cannot hold every "Sample Category"."Code" value (Code[20]).
-        // Compiles without a diagnostic; picking a category code longer than
-        // 10 characters through the lookup fails at runtime.
+        // Compiles without a diagnostic; assigning or validating a category code
+        // longer than 10 characters fails at runtime.
         field(2; "Category Code"; Code[10])
         {
             TableRelation = "Sample Category"."Code";