test: pin draft prototype-inspection behavior restored by #1271

[jonatankruszewski]

Follow-up to our conversation. This adds a test-only file that pins the prototype-inspection behavior of drafts, so it can't silently regress in a future 11.x refactor.

Why

#1259 wrapped constructor/__proto__ reads in a sanitizing Proxy. That broke ecosystem libraries which introspect prototypes to classify values — e.g. lodash's isPrototype (behind _.isEmpty/_.isEqual/_.isPlainObject) reads
value.constructor.prototype, and fast-deep-equal compares a.constructor.

Reading constructor.prototype through the guard tripped the Proxy invariant for Object's non-configurable prototype, so those reads threw or misreported

(#1265, #1266, #1268). It was reverted in #1271.

Right now nothing in the suite locks in the reverted behavior, so the same guard could get reintroduced without a failing test. These cases close that gap.

What's covered

• draft.constructor returns the real constructor for object, array, and [immerable] class drafts
• constructor identity stays stable across sibling drafts (Regression when comparing equality using fast-deep-equal #1266)
• draft.constructor.prototype is readable and doesn't throw (TypeError: 'get' on proxy: property 'prototype' is a read-only and non-configurable data property on the proxy target #1265, lodash.isEmpty() on a draft throws "TypeError: 'get' on proxy: property 'prototype'..." since 11.1.9 #1268)
• lodash's isPrototype idiom works on drafts (lodash.isEmpty() on a draft throws "TypeError: 'get' on proxy: property 'prototype'..." since 11.1.9 #1268)
• reserved names (constructor) report correctly through in / Object.keys, and own data keys named constructor / prototype round-trip
• Object.getPrototypeOf / __proto__ return the real prototype
• setPrototypeOf on a draft is still forbidden

Notes

• No source changes — __tests__/prototype-inspection.js only.
• The file has a header comment documenting the rationale (the fix: prevent prototype pollution via constructor.prototype access  #1259 → fix: Revert "prevent prototype pollution via constructor.prototype ac… #1271 history and the two consumer patterns), so the intent survives future refactors. Happy to trim it if you'd rather keep it lean.