Add foundational AL developer knowledge (#195)

Co-authored-by: Jesper Schulz-Wedde <jesper.schulzwedde@microsoft.com>
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

microsoft/knowledge/interfaces/guard-interface-casts-with-is.bad.al

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+interface "I Quote Amount Bad"
+{
+    procedure GetAmount(): Decimal;
+}
+
+interface "I Quote Date Bad"
+{
+    procedure GetDate(): Date;
+}
+
+codeunit 50611 "Quote Reader Bad"
+{
+    procedure GetDate(Quote: Interface "I Quote Amount Bad"): Date
+    var
+        DatedQuote: Interface "I Quote Date Bad";
+    begin
+        DatedQuote := Quote as "I Quote Date Bad";
+        exit(DatedQuote.GetDate());
+    end;
+}

microsoft/knowledge/interfaces/guard-interface-casts-with-is.good.al

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+interface "I Quote Amount Good"
+{
+    procedure GetAmount(): Decimal;
+}
+
+interface "I Quote Date Good"
+{
+    procedure GetDate(): Date;
+}
+
+codeunit 50610 "Quote Reader Good"
+{
+    procedure TryGetDate(Quote: Interface "I Quote Amount Good"; var QuoteDate: Date): Boolean
+    var
+        DatedQuote: Interface "I Quote Date Good";
+    begin
+        if not (Quote is "I Quote Date Good") then
+            exit(false);
+
+        DatedQuote := Quote as "I Quote Date Good";
+        QuoteDate := DatedQuote.GetDate();
+        exit(true);
+    end;
+}

microsoft/knowledge/interfaces/guard-interface-casts-with-is.md

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+---
+bc-version: [25..]
+domain: interfaces
+keywords: [interface, is-operator, as-operator, type-test, cast, variant, runtime-error]
+technologies: [al]
+countries: [w1]
+application-area: [all]
+---
+
+# Guard optional interface casts with `is`
+
+## Description
+
+From runtime 15.0, AL can type-test an interface or `Variant` with `is` and cast it to another interface with `as`. The test is non-throwing, but `as` raises a runtime error when the underlying codeunit does not implement the target interface. This matters when an extended capability is optional or implementations can come from other extensions.
+
+## Best Practice
+
+Use `is` to establish that the value supports the target interface before using `as`. Cast directly only where the target implementation is an invariant guaranteed by the surrounding contract.
+
+See sample: [`guard-interface-casts-with-is.good.al`](guard-interface-casts-with-is.good.al).
+
+## Anti Pattern
+
+Using `as` unconditionally for an optional extended interface. An otherwise valid implementation of the base interface then fails at runtime merely because it does not implement the additional contract.
+
+See sample: [`guard-interface-casts-with-is.bad.al`](guard-interface-casts-with-is.bad.al).
+
+## Reference
+
+[Understand type testing and casting operators for interfaces](https://learn.microsoft.com/en-us/training/modules/business-central-interfaces/type-testing)