fix(scorm2004): honor shared-data default

[jcputney]

Summary

• represent adlcp:sharedDataGlobalToSystem as nullable so attribute presence is preserved
• apply the SCORM 2004 default of true when the attribute is omitted
• retain an explicitly authored false value

Testing

• ./mvnw -Dtest=Scorm2004ManifestTest test
• ./mvnw test (1,245 tests, 0 failures)

[github-actions]

📊 Benchmark Results

Triggered by workflow run.

Benchmark	Score	Error	Unit
🟢 detectModuleType	0.001	±0.000	ms/op
🟢 parseModule	1.044	±2.293	ms/op
🟢 detectModuleType	0.000	±0.000	ms/op
🟢 parseModule	0.069	±0.055	ms/op
🟢 detectModuleType	0.159	±0.694	ms/op
🟢 parseModule	0.512	±0.902	ms/op
🟢 detectModuleType	0.100	±0.018	ms/op
🟢 parseModule	1.342	±3.638	ms/op

✅ All benchmarks passed performance thresholds.