fix: keep generator send values through ternary echoes

[nahime0]

Summary

• keep generator yield-assignment values live through ternary echo statements
• add Mixed-slot null checks to the narrow generator IR
• add a regression test for Generator::send() followed by a ternary echo before termination

Validation

• cargo test test_generator_send_value_reaches_ternary_echo_before_termination -- --nocapture
• cargo test codegen::generators::send_throw
• cargo test codegen::generators
• manual PHP/elephc comparison for the issue reproduction: both print 15
• git diff --check

Closes #308