feat: print partial report and exit 130 on SIGINT

[paveg]

Summary

When a load test is interrupted with Ctrl-C (SIGINT),
molt now prints the statistics collected so far and exits with code 130 (128 + SIGINT),
instead of dying without a report.

Since async 0.18, SIGINT/SIGTERM cancel the whole async program gracefully by default.
Previously that cancellation propagated straight out of coordinator.run()'s task group and killed the process reportless.

Changes

• coordinator.run() catches cancellation (via the public @async.is_cancellation_error predicate; any other task-group failure still propagates) and falls through to the existing finalize/report path, so partial stats are collected and printed. TUI terminal restore (on_cleanup) runs before the report, unchanged
• run() now returns RunOutcome { stats, interrupted } instead of bare Stats, keeping the interrupted state explicit without touching the shared Stats type
• main exits 130 when interrupted; this takes precedence over threshold (3) and failed-request (2) exit codes.
  Both signal-triggered and externally-induced cancellation flow through the same Cancelled error in the async runtime (verified in the moonbitlang/async source), so one mechanism covers both
• Also removes an unreachable panic() after @sys.exit(3) in the threshold path

Verification

• TDD: the new integration test (cancel coordinator.run mid-flight, assert partial stats are returned with interrupted=true) was confirmed failing to compile before the implementation existed
• moon test --target native 272/272, moon test (wasm-gc) 195/195, after rebasing onto current main
• End-to-end with the release binary: SIGINT sent 2s into a 30s run → full summary printed (55k requests collected) and exit code 130; an uninterrupted run still completes normally with exit 0

Known minor behavior: requests in flight at the moment of cancellation are recorded as Other(Cancelled) errors in the report (2 occurrences in the E2E run above). Left as-is in this PR to keep the change minimal.

🤖 Generated with Claude Code

[changeset-bot]

⚠️ No Changeset found

Latest commit: 478f40a

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.73%. Comparing base (f869166) to head (478f40a).

Files with missing lines	Patch %	Lines
src/cmd/main/main.mbt	0.00%	2 Missing ⚠️
src/lib/coordinator/coordinator.mbt	66.66%	1 Missing ⚠️
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #25      +/-   ##
==========================================
+ Coverage   70.81%   71.73%   +0.91%     
==========================================
  Files          16       16              
  Lines         747      750       +3     
==========================================
+ Hits          529      538       +9     
+ Misses        218      212       -6     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.