Original fictional permission policy. No object was requested.
An authorized read returns 200; a forbidden read returns 403.
Only the owner may read a private row. Shared rows may be read by A or B.
Every caller in the matrix is already authenticated.

Worked case: B reads shared row 9. The sharing rule allows this read.
Activity evidence: A reads private row 7, owner A. Compare reader and owner.
Checkpoint evidence is stated in the question. Decide it independently.