28edf7c75a made stage() chmod the native agent-browser binary, but
entries staged before it (lazy installs since the pm store landed) stay
0644 forever: every browser_* call fails with PermissionError and the
store never restages because the entry verifies. Restore the bit in
place when the entry is looked up on its own target (modes are not part
of the pinned digest); an entry that cannot be chmodded counts as not
installed so ensure() restages it.
92ce6f4760