test: expand black-box coverage probes

[bobzhang]

Summary

• add black-box coverage tests across core packages
• add JS-only tests for wrappers/conversions (blocked by JS coverage ICE)
• document remaining coverage gaps and target constraints in coverage/coverage_gaps.md

Coverage

• wasm-gc: moon coverage analyze -- -f summary
• js: moon test --target js --enable-coverage (moonc v0.6.37+0b3e4ae80 ICE)

Notes

• remaining uncovered lines are categorized in coverage/coverage_gaps.md

[coveralls]

Pull Request Test Coverage Report for Build 2244

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 98.344%

---

Totals
Change from base Build 2240:	0.1%
Covered Lines:	10629
Relevant Lines:	10808

---

💛 - Coveralls