Filling the bench writes squad rows behind Core's back, which invalidates the
stored FIFA17 opaque extension: Core saw a canonical squad that no longer
matched the extension's fingerprint, the host refused to apply it
(`stale_integrity`), and the client got a squad with zero players and no
manager. Nothing failed loudly — the rig just came up empty.
The seeder now recomputes the fingerprint exactly as
`services::squad::squad_fingerprint` does, and bring-up asserts the squad really
projects (>= 18 occupied, a manager present) instead of trusting that it did.
Seeded owned rows also state their content_kind, so Core does not record a kit
or a manager as a player.
funman300
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