Additional test coverage for nested .await check() injection

[rocketman-code]

Context

PR #145 added recursive check() injection for nested .await expressions with 10 unit tests. Code review identified two additional test cases that would strengthen coverage.

Additional tests

1. let x = foo().await; inside nested blocks (e.g., inside if, match) -- the common let binding pattern is covered by the general logic but has no explicit test
2. else if with .await in the then-branch -- handled correctly by syn's ExprIf traversal of else_branch, but an explicit test would document the guarantee since ExprWhile vs ExprIf are different AST nodes

Priority

Low -- existing implementation handles both cases correctly. These are documentation-value tests that guard against future regressions if the visitor overrides change.