PHLEGM v8 compiler progress basis update

- After a successful build, persist its actual elapsed duration in the existing external Compiler UI settings.
- The next build uses that last-good duration as the determinate progress basis, capped below 100% until the process exits.
- Updated in checkpoint 33: failures and cancellations never affect the most recent N successful duration samples; use Unknown Duration only when the history is empty.
- With no prior completed build, Unknown Duration is used because no successful duration basis exists yet.
- A failed/cancelled build does not destroy the older last-good duration; a later successful build replaces it.
- Build history persists across Compiler UI launches and clean Developer Distribution re-extractions alongside the existing saved compiler selections.
