Potential fix for code scanning alert no. 23: Prototype-polluting function

[NullSablex]

Potential fix for https://github.com/NullSablex/PawnPro/security/code-scanning/23

General fix: in deep-assignment code, enforce an explicit denylist for dangerous keys (__proto__, constructor, prototype) at the assignment/traversal point, and ensure traversal only proceeds through own plain-object properties.

Best targeted fix in src/core/config.ts:

1. In setKey, after splitting dotPath, reject empty segments and reject dangerous keys with a local guard.
2. In traversal, use Object.prototype.hasOwnProperty.call(cursor, key) + isPlainObject before descending; otherwise create a new plain object.
3. Before writing leaf, apply the same explicit dangerous-key block.
4. Keep existing behavior otherwise unchanged.

No new imports/dependencies are needed.

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