Replace for-in with Object.keys loop in Node.init and cloneNode

[alanzabihi]

Hypothesis

Node.init() (node.js) uses for-in with hasOwnProperty to iterate over defaults object properties. cloneNode() (node.js) also uses for-in + hasOwnProperty for deep cloning. V8 optimizes for-in poorly compared to Object.keys() + indexed for loop because for-in must check the prototype chain and handle non-enumerable properties.

Replace for-in patterns with:

let keys = Object.keys(defaults)
for (let i = 0; i < keys.length; i++) {
  let key = keys[i]
  // ... assignment
}

This gives V8 a tight indexed loop it can JIT-optimize aggressively. Object.keys() returns only own enumerable properties, making the hasOwnProperty check unnecessary.

Also in cloneNode, replace the recursive for-in deep copy with a structured approach using Object.keys.

Editable surface

• lib/node.js — replace for-in with Object.keys in init() and cloneNode()

What's different from prior work

• Inline hot-path node creation in Parser to avoid constructor overhead #10 tried Object.create() to bypass constructors entirely (couldnt verify). This thesis keeps constructors but optimizes the iteration pattern inside them.
• Reduce Node constructor property initialization overhead #26 tried reducing constructor property initialization (no improvement). This specifically targets the for-in vs Object.keys performance gap.
• Optimize Node.clone() to avoid JSON round-trip for deep copy #16 tried Object.keys in cloneNode but combined with other changes and ran under heavy load.

Expected impact

METRIC_A improvement of 1-3ms (thousands of node creations). METRIC_B improvement of 2-4ms. Combined 2-4ms.

[alanzabihi]

Polyresearch approval: thesis #70.