safety: gate sp_detour_on behind 'force' (EncodeField-entry detour crashes)

The entry detour reads stack args the 6-arg cdecl passthrough doesn't preserve
-> crashes after ~8 calls (confirmed live). It already proved managed
interception + revealed args; real hook is field+0x38. Gated so it can't be
tripped accidentally.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>