Keep Newton division divisor alive

[OskarEichler]

Summary:

• correct the second duplicated RB_GC_GUARD for a2 to guard b2
• retain the divisor wrapper through the end of VpDivdNewtonInner

Why:
The method constructs a2, b2, c2 and r2, but its final guards name a2 twice and omit b2. The duplicate is a maintenance/liveness typo at a native GC boundary.

Verification:

• full current suite: 266 tests, 13,616 assertions, 0 failures/errors/omissions
• RBS suite: 146 tests, 1,805 assertions, 0 failures/errors
• 25 GC.stress division comparisons pass before and after the correction

Compatibility:
No arithmetic behavior changes. This makes the existing lifetime intent explicit; the stress model did not reproduce a user-visible crash on the tested runtime.