fix(data): official DfE KS4 national headline averages; drop mislabelled computed means

New ees_ks4_national stream ingests the EES 'National characteristics
summary data' series (England, state-funded, all pupils). The old mart's
unweighted school means were 7-15 points off every headline measure and
produced an impossible national Progress 8 (-0.27). The API's computed
fallback is gone too: the footnote calls these figures official, so an
unbuilt mart now yields an empty series, never a stand-in.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0146VHeLAWjDVE2B5uU67jCB

pipeline/transform/target/osi_document.json

@@ -0,0 +1 @@
+{"version": "0.1.1", "dialects": ["ANSI_SQL"], "semantic_model": [{"name": "semantic_model", "datasets": []}]}