Add foundational AL developer knowledge

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

microsoft/knowledge/data-modeling/round-direction-symbols-use-magnitude.bad.al

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+codeunit 50601 "Directed Rounding Bad"
+{
+    procedure FloorAmount(Value: Decimal; Precision: Decimal): Decimal
+    begin
+        // For negative values, '<' rounds toward zero rather than toward negative infinity.
+        exit(Round(Value, Precision, '<'));
+    end;
+}