When both arms of an if-statement prove the same variable non-null
(via M22 null-check inference or M23 address-of marking), the
summary encoder's `mergeBindings` now intersects the two branches'
nonNullVars sets and marks the survivors in the post-if env.
Mirrors the same logic already present in
security_walker.cpp::walkIf so summary mode and the main walker
stay in parity.
Added tests/unit/test_summary_encoder_phi_nonnull.cpp covering
the rename-pair address-of case. Pre-fix the test FAILs; post-fix
PASSes. 44/44 still green.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
