fix: make dead_lettering.cr compile standalone

[viktorerlingsson]

The CI job that compiles each changed *.cr file independently has been failing on src/lavinmq/amqp/argument/dead_lettering.cr:

Error: undefined constant AMQP::Queue

The file references AMQP::Queue in alias Task = {AMQP::Queue, Message} | MessageRoutedCallback but never requires it. It only builds as part of the full program because queue.cr requires it.

Requiring ../queue/queue at the top doesn't work - queue.cr does include Argument::DeadLettering, and an include needs the module body already processed, so entering from dead_lettering.cr just fails the other way (undefined constant Argument::DeadLettering). Requiring it after the module is defined satisfies both entry points: the in-progress require is a no-op on the second pass, and the alias target resolves in the later semantic phase.

Note this wasn't introduced by 8256eff - the file already failed standalone before it. That commit was just the first to touch the file after the changed-files check landed, which is where it surfaced.

Test plan

• crystal build --no-codegen --error-on-warnings src/lavinmq/amqp/argument/dead_lettering.cr passes
• Same sweep over every src/**/*.cr - this was the only failure repo-wide
• crystal build --no-codegen --error-on-warnings src/lavinmq.cr, make lint, crystal tool format --check src/ all clean
• make test green

🤖 Generated with Claude Code

[claude]

No issues found.

The change adds require "../queue/queue" at the end of dead_lettering.cr to break a circular require (queue.cr:18 requires dead_lettering, and DeadLettering references AMQP::Queue). Placing the require after the module definition is the correct idiom for this — Crystal processes each require once, so it works whether dead_lettering.cr or queue.cr is the compilation entry point. The path resolves correctly. No behavioral change, so no spec is warranted.