dillon1000/react

[Flight] Walk parsed JSON instead of using reviver for parsing RSC payload (#35776)

## Summary

Follow-up to https://github.com/vercel/next.js/pull/89823 with the
actual changes to React.

Replaces the `JSON.parse` reviver callback in `initializeModelChunk`
with a two-step approach: plain `JSON.parse()` followed by a recursive
`reviveModel()` post-process (same as in Flight Reply Server). This
yields a **~75% speedup** in RSC chunk deserialization.

| Payload | Original (ms) | Walk (ms) | Speedup |
|---------|---------------|-----------|---------|
| Small (2 elements, 142B) | 0.0024 | 0.0007 | **+72%** |
| Medium (~12 elements, 914B) | 0.0116 | 0.0031 | **+73%** |
| Large (~90 elements, 16.7KB) | 0.1836 | 0.0451 | **+75%** |
| XL (~200 elements, 25.7KB) | 0.3742 | 0.0913 | **+76%** |
| Table (1000 rows, 110KB) | 3.0862 | 0.6887 | **+78%** |

## Problem

`createFromJSONCallback` returns a reviver function passed as the second
argument to `JSON.parse()`. This reviver is called for **every key-value
pair** in the parsed JSON. While the logic inside the reviver is
lightweight, the dominant cost is the **C++ → JavaScript boundary
crossing** — V8's `JSON.parse` is implemented in C++, and calling back
into JavaScript for every node incurs significant overhead.

Even a trivial no-op reviver `(k, v) => v` makes `JSON.parse` **~4x
slower** than bare `JSON.parse` without a reviver:

```
108 KB payload:
  Bare JSON.parse:    0.60 ms
  Trivial reviver:    2.95 ms  (+391%)
```

## Change

Replace the reviver with a two-step process:

1. `JSON.parse(resolvedModel)` — parse the entire payload in C++ with no
callbacks
2. `reviveModel` — recursively walk the resulting object in pure
JavaScript to apply RSC transformations

The `reviveModel` function includes additional optimizations over the
original reviver:
- **Short-circuits plain strings**: only calls `parseModelString` when
the string starts with `$`, skipping the vast majority of strings (class
names, text content, etc.)
- **Stays entirely in JavaScript** — no C++ boundary crossings during
the walk

## Results

You can find the related applications in the [Next.js PR
](https://github.com/vercel/next.js/pull/89823)as I've been testing this
on Next.js applications.

### Table as Server Component with 1000 items

Before:

```
    "min": 13.782875000000786,
    "max": 22.23400000000038,
    "avg": 17.116868530000083,
    "p50": 17.10766700000022,
    "p75": 18.50787499999933,
    "p95": 20.426249999998618,
    "p99": 21.814125000000786
```

After:

```
    "min": 10.963916999999128,
    "max": 18.096083000000363,
    "avg": 13.543286884999988,
    "p50": 13.58350000000064,
    "p75": 14.871791999999914,
    "p95": 16.08429099999921,
    "p99": 17.591458000000785
```

### Table as Client Component with 1000 items

Before:

```
    "min": 3.888875000000553,
    "max": 9.044959000000745,
    "avg": 4.651271475000067,
    "p50": 4.555749999999534,
    "p75": 4.966624999999112,
    "p95": 5.47754200000054,
    "p99": 6.109499999998661
````

After:

```
    "min": 3.5986250000005384,
    "max": 5.374291000000085,
    "avg": 4.142990245000046,
    "p50": 4.10570799999914,
    "p75": 4.392041999999492,
    "p95": 4.740084000000934,
    "p99": 5.1652500000000146
```

### Nested Suspense

Before:

```
  Requests:  200
  Min:       73ms
  Max:       106ms
  Avg:       78ms
  P50:       77ms
  P75:       80ms
  P95:       85ms
  P99:       94ms
```

After:

```
  Requests:  200
  Min:       56ms
  Max:       67ms
  Avg:       59ms
  P50:       58ms
  P75:       60ms
  P95:       65ms
  P99:       66ms
```

### Even more nested Suspense (double-level Suspense)

Before:

```
  Requests:  200
  Min:       159ms
  Max:       208ms
  Avg:       169ms
  P50:       167ms
  P75:       173ms
  P95:       183ms
  P99:       188ms
```

After:

```
  Requests:  200
  Min:       125ms
  Max:       170ms
  Avg:       134ms
  P50:       132ms
  P75:       138ms
  P95:       148ms
  P99:       160ms
```

## How did you test this change?

Ran it across many Next.js benchmark applications.

The entire Next.js test suite passes with this change.

---------

Co-authored-by: Hendrik Liebau <[email protected]>