docs: survey_weight is col 28 under schema v6 (was col 26 in v5)

Rebase onto the v6 main (bd53230) shifted survey_weight from col 26 to
col 28: v5→v6 inserted cog_legacy_state/cog_legacy_county at positions
10-11 (26→28 cols). Position confirmed against the regenerated v6
fixture and both corpus docs (reader-specification.md §3 'Long parquet
schema (28 columns)' row 28; data_dictionary.md '28-column schema v6'
row 28).

README.md

@@ -28,7 +28,7 @@ package implements.
 ## Raw-parquet caveat: `survey_weight` is not an aggregation weight
 
 Users reading the corpus parquet directly (DuckDB, arrow) will see a
-`survey_weight` column (schema v5, col 26). It is legacy Census IndFin
+`survey_weight` column (schema v6, col 28). It is legacy Census IndFin
 sample-design **metadata passed through verbatim** — the Census Bureau's own
 source documentation says it "is for informational purposes only and should
 not be used to derive any other statistics" (`_ReadMe_First_IndFin.txt`;