Fix NaturalArray assignment recursion causing LazyLoop stack overflow (without changing LazyLoop behavior)

[Copilot]

LazyLoop.cod was failing due to a runtime recursion path in natural-array assignment, and the previous workaround altered demo behavior. This change restores the original LazyLoop content and fixes the underlying engine issue so the demo can run as authored.

• Runtime fix: avoid formula-triggering reads during assignment bookkeeping

  • Added NaturalArray.peekMaterialized(long index) to read only already-materialized values (cache / computedCache) without invoking formula evaluation.
  • Updated AssignmentHandler natural-array index assignment to use peekMaterialized(...) for previous value tracking before set(...).

• Restore demo semantics

  • Reverted src/main/cod/demo/src/main/test/lazyloop/LazyLoop.cod to its original output checks (arr[2], arr[3], arr[24000], arr[24001]), removing the earlier behavior change.

• Documentation alignment

  • Updated README wording to reflect that the fix is in runtime recursion handling, not in removing LazyLoop output lines.

// before
Object previous = natural.get(index);
natural.set(index, newValue);

// after
Object previous = natural.peekMaterialized(index);
natural.set(index, newValue);