Fix prototype-named code modules

[junaga]

What changed

• Store require.cache and require.initGlobals in null-prototype dictionaries.
• Check both the cache and uploaded module map with an own-property test.
• Add regressions for constructor, toString, and __proto__ module names, plus the corresponding unknown-module case.

Why

The loader used moduleName in require.cache while the cache inherited from Object.prototype. A module named constructor or toString was therefore treated as already cached and returned the inherited built-in instead of evaluating player code. __proto__ also could not be stored safely through ordinary object assignment.

Using dictionary objects and own-property checks makes every uploaded module name behave consistently and prevents inherited properties from masquerading as modules.

Validation

• Full Jasmine suite: 112 specs, 0 failures
• Gulp build completed successfully