Support separate error channel in StreamPrinter for imix

[KCarretto]

Updated StreamPrinter in imix to support a separate error channel (error_tx). This allows output from eprint! (and equivalent Eldritch built-ins) to be captured and reported as TaskErrors in the TaskOutput structure, rather than being mixed with standard output. A new test case confirms that eprint messages are correctly routed to the error field.

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PR created automatically by Jules for task 6216986745705561755 started by @KCarretto

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[github-actions]

Summary

Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
2615    ±0	2615    ±0	0    ±0	0    ±0	0    ±0	0    ±0	1ms    ±0

Previous Results

Build 🏗️	Result 🧪	Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
#1041	✅	2615	2615	0	0	0	0	36.2s

Insights

Average Tests per Run	Total Flaky Tests	Total Failed	Slowest Test (p95)
2615	0	0	5.2s

Slowest Tests

Test 📝	Results 📊	Duration (avg) ⏱️	Duration (p95) ⏱️
eldritch-libsys: std::dll_inject_impl::tests::test_dll_inject_simple	1	5.2s	5.2s
TestDockerExecutor_Build_ContextCancellation	1	5.1s	5.1s
TestInteractiveShell	1	5.1s	5.1s
TestOtherStreamOutput	1	5.0s	5.0s
imix::bin/imix: install::tests::test_install_execution	3	1.2s	3.5s
imix::bin/imix: install::tests::test_install_execution	3	1.2s	3.5s
imix::bin/imix: install::tests::test_install_execution	3	1.2s	3.5s
imix::bin/imix: tests::task_tests::test_task_eprint_behavior	3	3.0s	3.0s
imix::bin/imix: tests::task_tests::test_task_eprint_behavior	3	3.0s	3.0s
imix::bin/imix: tests::task_tests::test_task_eprint_behavior	3	3.0s	3.0s

_{🎉 No failed tests in this run. | 🍂 No flaky tests in this run.}

Github Test Reporter by CTRF 💚

🔄 This comment has been updated