feat(gnovm): implement gno test -cover for code coverage reporting

[lamb356]

Summary

Implements code coverage reporting for gno test with three new flags:

• -cover - Enable coverage analysis
• -coverprofile=<file> - Write coverage profile to file
• -covermode=<mode> - Set coverage mode (set, count, or atomic)

Closes #1121
Closes #2907
Closes #2908

Changes

• Add -cover, -covermode, -coverprofile flags to gno test command
• Add coverage.go with CoverageCollector, CoverageData, CoverBlock types
• Track statement execution during test runs via op_exec.go
• Output Go-compatible coverage profiles (works with go tool cover)
• Add 40 unit tests with >80% coverage on all new code

Usage

gno test -cover ./examples/gno.land/p/demo/avl
# Output: coverage: 88.4% of statements

gno test -cover -coverprofile=coverage.out ./path/to/package

Testing

• 40 unit tests added

• 80% coverage on all new code

• Output compatible with Go's coverage profile format

[Gno2D2]

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