perf(object): skip array-key interning for non-arrays

[Eric-Song-Nop]

Stacked on #5. Base: perf/profilor; head: perf/array-own-key-early-check, both in pocket-stack/quickjs-oxide. Review and merge in dependency order.

Migrated from Eric-Song-Nop#1 with identical commits and preserved measurement/validation evidence.

修改 / Change

Generic property operations call array_own_key for ordinary objects too. Previously those calls interned "length" before returning Other. Check the payload first so non-arrays skip that work. End the state borrow before interning to avoid a RefCell conflict; preserve genuine Array length/index classification.

通用属性操作也会对普通对象调用 array_own_key。此前非数组在返回 Other 前会无用地 intern "length"；现在先检查 payload，非数组立即返回。intern 前结束 state 借用，避免 RefCell 冲突，保留真正数组的 length/index 分类。

Related issue / 关联 issue: #6

实测 / Measurements

Same PocketLab machine, matching release build flags, diagnostics disabled, five repetitions per version/workload with alternating order. All 20 runs passed output checks. / 同一 PocketLab 机器、相同 release 参数、关闭诊断，每版本每工作负载五次交替运行，20 次输出校验全部通过。

指标 / Metric	Before	After	Change
Fixed Richards ×10, median process ms / 固定工作量中位进程耗时	2411.234	2167.421	−10.11%
Original Richards harness, median process ms / 原 harness 中位进程耗时	9627.840	8726.416	−9.36%
Original Richards median score / 原 harness 中位分数	14.7	16.2	+10.20%

Whole-process times include startup/compilation/teardown; harness scores and fixed-workload times are separate metrics. This is one machine/workload family, not a general guarantee; CPU stacks and array-heavy performance were not remeasured. / 进程耗时包括启动、编译和收尾；harness 分数与固定工作量耗时是不同指标。这是单机单类工作负载结果，不是普遍收益保证；本次未重新采集 CPU 栈或数组密集型性能。

Report / 报告 · All samples, outputs and hashes / 全部样本、输出和哈希

验证 / Validation

• Formatting passed / 格式检查通过。
• Rust 1.88 engine library tests with profiling: 1,885 passed, 0 failed, including existing array/property/GC coverage. / 包含已有数组、属性及 GC 测试。
• Rust 1.88 strict production-target Clippy (--lib --bins --features profiling): passed / 通过。
• Initial broader all-targets strict Clippy failed on three unused imports in unchanged context/string-test source; logs retained. / 初次所有目标 strict Clippy 因未修改代码中的三处未使用导入失败，已保留日志。
• Full Test262 was not rerun for this change. / 本次未重跑完整 Test262。