The gdp-population pin's Yugoslav SFR series is 16.1% above FAOSTAT's own figure for the same aggregate-year

Split out of #862 and #863, where two agents reached this independently from different reference points. Filing so both can close on merge without losing it.

## The finding

For **area 248 (Yugoslav SFR), 1990**:

| source | population |
|---|--:|
| `gdp-population` pin | 26,487,416 |
| FAOSTAT FBS item 2501 / element 511 | 22,817,000 |
| sum of the six ISO3 codes the successor walk reaches | 21,840,518 |
| UN WPP `XKX` (Kosovo), the walk's known omission | 1,973,790 |

The pin is **16.1% above FAOSTAT** for the same aggregate and year. The successor sum sits within **4.3% of FAOSTAT**.

Measured independently from the other direction: of the pin's 17.5% gap against the successor sum, Kosovo accounts for **42%**; the residual **10.1%** is the pin's Yugoslav level disagreeing with WPP's own seven territories. The other four dissolved-federation aggregates agree to **1.5%**, so this is specific to this series.

## Why it matters

#863 attributes its 17.5% shortfall to the missing Kosovo code. That attribution is **at most partly right** — most of the gap looks like a property of the pin's `YUG` series rather than of the successor walk. Anyone tuning the walk to close 17.5% would be fitting to a bad reference.

## What done looks like

- Establish what the pin's `YUG` series actually is (which vintage, which source, whether it is a WHEP-side reconstruction) and why it exceeds both FAOSTAT and the sum of WPP's own territories.
- Decide whether it should be corrected, superseded, or documented as a known divergence.
- If it is corrected, re-derive #863's figures against the new baseline.

The other four aggregates agreeing to 1.5% is the strongest hint that this is one series, not a systematic issue.

Refs #862, #863, PR #922, PR #924.

Start by tracing the gdp-population pin's YUG series for area 248 in 1990 and compare its vintage or source with FAOSTAT FBS item 2501/element 511 and UN WPP's seven territories. Determine whether the discrepancy calls for correction, supersession, or documentation, and if corrected, re-derive #863's figures. Done means the YUG provenance and cause are established and the chosen resolution is applied consistently.

Written by the indexing model from the issue text.