fix(isIn): check object membership without calling shadowed methods

[FanWu-ai]

isIn supports object own-key membership, but it calls the object's hasOwnProperty value as a method. A null-prototype dictionary throws; a dictionary containing a hasOwnProperty key can throw or report incorrect membership.

Use Object.prototype.hasOwnProperty.call, matching the existing array branch. Add three regressions covering null prototypes, a non-callable method-name key, and an overridden method that falsely reports every key as present. Inherited keys remain excluded. No dependencies, generated distributions, or unrelated behavior are changed.

Validation on Linux:

• Pristine full npm test: 323 passing on Node 24.19.0, including builds, minification, ESLint and coverage.
• The three added regressions fail on unchanged source and pass after the fix.
• Final full npm test: 326 passing on both Node 24.19.0 and 22.19.0; all build/lint gates pass. Coverage is unchanged (100% statements/functions/lines; 96.59% branches).
• 3,390 additional checks across source, CommonJS, ES-module and browser distributions cover prototypes, shadowed keys, property descriptors, frozen objects, and array/string compatibility.
• git diff --check passes. Other CI runtimes were not run locally; hosted CI results are separate.

Object membership is established by the existing object tests and PR #336.

Checklist

• Changes are limited to the object-membership fix and its tests.
• README reviewed; no documented API or options changed.
• Regression tests included.
• Existing API reference linked above.

Prepared and tested with OpenAI Codex assistance. No human review or hosted CI pass is claimed.