docs/technical: control_twin_crossover_endpoint (technical ledger behind beatrix-ft2, verified 2026-09-04)

docs/technical/05_control_twin_crossover_endpoint.md

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-<!-- Technical ledger extracted 2026-09-04 from the deep-rewrite of the beatrix-ft2 article (every number verified against its ledger and sources by the ft2-finish pass). This is the technical record behind the human-readable article; it is not the article. -->
+<!-- Technical ledger extracted 2026-09-04 from the verified deep rewrite behind the beatrix-ft2 article (every number checked against its ledger and sources). This is the technical record; the human-readable article summarizes it. -->
 
 ## 5. The crossover