perf(v2): reduce resolver allocations

[harlan-zw]

Summary

Backports #812.

• reuse static arrays and objects while walking resolver input
• shallow copy only after a resolver or function changes a branch
• preserve isolated copies for v2's mutable Schema.org resolver markers
• drop unsafe prototype keys from copied results
• keep the shared prototype key guard internal

The upstream benchmark measured roughly 18% lower SSR allocation in this path. No exports, options, or public types are added.

Validation

• workspace suite: 707 passed, 2 skipped
• Unhead suite: 427 passed, 2 skipped
• Schema.org duplicate action regression: 9 passed
• scoped ESLint
• pnpm typecheck
• pnpm build

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