hotfix: lower generator concat echoes after yield from

[nahime0]

Summary

• lower generator echo concat expressions as source-order echo operations
• keep variables used in generator concat echoes on the Mixed path
• add a regression for yield-from return values used in concat echoes

Validation

• cargo test test_generator_yield_from_return_value_survives_concat_echo_after_delegation -- --nocapture
• cargo test codegen::generators::yield_from
• cargo test codegen::generators
• manual PHP/elephc comparison for the issue reproduction: both print 1:9
• git diff --check

Closes #192