Preserve native locked-thread list ownership

[OskarEichler]

Summary:

• include the tail node when checking locked-thread membership
• move locked_tail when removing the final node through its predecessor

Why:
Membership iteration stops before locked_tail, so re-adding the tail thread inserts a duplicate. Removing a tail through the middle-list path also leaves locked_tail pointing to freed memory. Subsequent additions can write through stale native state.

Verification:

• focused native model covers duplicate tail insertion plus tail removal/replacement
• current composite suite remains at upstream baseline: 522 runs, 770 assertions, 13 known Ruby 4 layout failures, 0 errors, 2 skips
• native extension/gem build and syntax checks pass

Compatibility:
Lock ordering and public thread behavior are unchanged; the internal singly-linked list now maintains its membership and tail invariants.