Fix $watch oldValue for objects

[calebporzio]

Summary

• Bug: When deep-watching objects with $watch, oldValue was the same reference as value. By callback time, both reflected the already-mutated state. oldValue.bar showed 'bob' instead of 'baz'.
• Fix: Reuse the JSON.stringify result already computed for deep tracking (line 68), store it, and JSON.parse it back as oldValue when the callback fires. Zero additional stringify calls.
• Tests: 3 new tests — objects, arrays, and null→object transition. All 11 $watch tests pass.

The bug

$watch('foo', (value, oldValue) => {
    console.log(value.bar)    // 'bob'
    console.log(oldValue.bar) // Expected 'baz', got 'bob' ❌
})

oldValue = value stores a proxy reference. For objects, both point to the same reactive proxy — the old state is lost.

The fix

- JSON.stringify(value)
+ let newJSON = JSON.stringify(value)

- let previousValue = oldValue
+ let previousValue = typeof oldValue === 'object'
+     ? JSON.parse(oldValueJSON)
+     : oldValue

  oldValue = value
+ oldValueJSON = newJSON

Follows entangle.js precedent (JSON.parse(JSON.stringify())). Same JSON edge-case limitations already exist on line 68.

Closes #3293. Related: #4406 (original PR by @Suall1969 who identified the fix direction), #3046, #3617.

Test plan

• deep $watch receives old value — object nested property change
• deep $watch receives old value for arrays — array push mutation
• deep $watch receives old value with null — null to object transition
• All 8 existing $watch tests still pass
• entangle and x-data tests pass (no regressions)

🤖 Generated with Claude Code