Support custom functions downstream of iterative (circular) calculation - currently return #CALC!

Office.js custom functions in Excel return #CALC! whenever any of their precedents involve a circular reference chain resolved by iterative calculation, even when the function itself is not part of the loop. (Tracked in https://github.com/OfficeDev/office-js/issues/4738, open since July 2024.)

Many common financial usecases require iterative calculations as part of standard practices. As an example, average-balance interest with iterative calculation is the mainstream convention in leveraged-finance modeling. Interest expense depends on the average debt balance, which depends on cash paydown, which depends on interest expense. Analysts across banking and private equity resolve this loop with Excel's iterative calculation setting. The same pattern appears in revolver/cash-sweep modeling and fund fee/NAV calculations. These workbooks worked for decades with XLL and VBA UDFs.

Any custom function consuming a model's outputs (e.g. an add-in function that writes results back to an external platform) is by definition downstream of the circular chain, so it always fails in these models. We've verified: a custom function with literal arguments works; the same function with an argument fed from the converged loop returns #CALC!; severing the dependency with IF(TRUE, value, FN(...)) makes it work again. This blocks migration from legacy XLL/VBA add-ins to the modern Custom Functions API for financial-modeling users.

The ask: custom functions downstream of a circular chain should evaluate against the loop's converged values after the iteration pass completes — matching legacy XLL/VBA UDF behavior. Custom functions don't need to participate in the iteration itself.