fix: map the app's cache group like its real group

[oonid]

In the app, /tmp is the app's cache dir, which Android makes setgid to the app's cache group (u0_a515_cache, gid 20515). Files created there showed that gid in a fake-root session (ls -ln /tmp → 0 20515), because the owner rule (#9) mapped only the app's primary gid. Found while verifying #9.

• fake_id0 computes the cache gid at initialization (AOSP multiuser_get_cache_gid: app id - 10000 + 20000) and keeps it in Config.app_cache_gid only if PRoot's process is in that group. An app process is; run-as, the VM and desktop Linux get (gid_t) -1, so nothing changes there.
• fake_owner_gid() maps it like the real gid, so stat, statx and the USERLAND fstat path show the fake group. The gid and the constants match bionic's u0_aNNNN_cache mapping (grp_pwd.cpp).
• New fakeroot test: a new file in /tmp has geteuid():getegid(). Docs: proot-improvement.md §35, counts 54 → 55.

Verification

• Android 16 (Samsung), in-app terminal (the cache dir is /tmp only there): before the fix the new test fails (/tmp/pit-fr-tmp-group is 0:20515, want 0:0); after it fakeroot 7/7 and ls -ln /tmp/pit shows 0 0. pit all from in-app terminals 55/55 on alpine and ubuntu; pr-cli test alpine and pr-cli test ubuntu 55/55.
• Alpine arm64 VM (mapping disabled there): alpine 51/51, .l2s 51/51 (ssh skipped), Ubuntu 26.04 55/55, fakeroot without link2symlink 7/7; as a uid != gid user general 9/9, chroot 2/2, fakeroot 7/7.
• Host cargo test 169; every commit builds with only the 8 upstream warnings.
• Committed libproot.so and libpr-cli.so rebuild bit-identically from the branch.

Found, not changed

Directories created in the app's /tmp inherit the cache dir's setgid bit (drwxr-sr-x). Mode bits are not faked: the bit is real and decides the group of files created inside.