[jayhemnani9910]

Fixes #342

next_uid and next_gid take a system flag. For system accounts the next ID is one below the lowest system ID in use, starting at SYS_UID_MAX / SYS_GID_MAX, and once that reaches the bottom of the range, the highest free ID. Regular accounts keep the current rule. useradd -r, groupadd -r and newusers -r pass their flag through.

test_uid_allocation_system expected 101 (the lowest ID); it now expects 999, which is what GNU gives on an empty range.

Tests: unit tests for the countdown and the gap fallback, and root tests creating two system users and two system groups (999, then 998). All fail before, pass after. cargo test --workspace run as root in an Ubuntu 26.04 container: 947 passed (943 on main, plus the four new). cargo fmt --check and cargo clippy --workspace --all-targets -- -D warnings are clean; the pam configurations were not run locally. Against GNU shadow 4.17.4 in the same container, 12 command sequences (useradd -r, groupadd -r, -u 500, an existing 998, a deleted system user, -U, -N, and regular users) now give the same files and exit codes.

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