refactor: groundwork for TYPE — sizeof constants, string guard, array scoping

- Element sizes now flow as target-folded null-GEP sizeof constants
  (TypeSystem::getSizeOfConstant); the module has no DataLayout string,
  so host-side size math would diverge from the backend on padded structs.
- Extract the triplicated STRING null->"" select into
  CodeGen::emitStringNullGuard.
- Scope ArrayTable per function via ArrayHandler::exchangeTable, mirroring
  the Symbols save/clear/restore discipline.
- Promote checked element addressing to public
  ArrayHandler::emitElementAddress for the upcoming lvalue layer.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

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