fix(coverage): break import cycle with debug

[bobzhang]

Summary

coverage/debug.mbt (added in #3482) introduced a Debug impl for CoverageCounter, which made coverage import debug. With --enable-coverage, the toolchain implicitly injects coverage into every package — including debug itself — producing the cycle:

abort -> prelude -> coverage -> debug -> coverage

This causes moon test --enable-coverage (and the misc-check CI job) to fail with Import loop detected.

Fix

• Delete coverage/debug.mbt.
• Remove "moonbitlang/core/debug" from coverage/moon.pkg.

The only observable API change is that CoverageCounter no longer has a Debug impl — it's an internal counter type that callers shouldn't be debug-printing in normal code.

Repro

$ git checkout origin/main
$ moon test --enable-coverage
Error: Failed to solve package relationship
Caused by:
    Error 1: Import loop detected: moonbitlang/core/abort -> moonbitlang/core/prelude -> moonbitlang/core/coverage -> moonbitlang/core/debug -> moonbitlang/core/coverage

Test plan

• moon test --enable-coverage — 6369/6369 pass after the fix.
• moon test — 6364/6364 pass.

mbti diff (coverage)

< import { "moonbitlang/core/debug" }
< pub impl @debug.Debug for CoverageCounter

🤖 Generated with Claude Code

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.

[coveralls]

Coverage Report for CI Build 4042

Warning

No base build found for commit bf02241 on main.
Coverage changes can't be calculated without a base build.
If a base build is processing, this comment will update automatically when it completes.

Coverage: 94.817%

Details

• Patch coverage: No coverable lines changed in this PR.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

Requires a base build to compare against. How to fix this →

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Coverage Stats

Relevant Lines:	15589
Covered Lines:	14781
Line Coverage:	94.82%
Coverage Strength:	220117.59 hits per line

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💛 - Coveralls