Mimalloc v3 memory usage regression

Nobody has claimed this yet.

We use mimalloc in [Bun](https://github.com/oven-sh/bun) and locally testing mimalloc `dev3` in several of our benchmarks yielded a positive result which is exciting, but a handful of our memory leak stress tests in CI regress. I'm mostly filing this issue so you have another data point for the v3 branch - not asking you to fix a bug or anything. We will stick to v2 for now but will give v3 another go in the future.

Logs from this test:
```ts
  | 2025-02-16 02:22:19 PST | Start RSS 2704
  | 2025-02-16 02:22:19 PST | RSS Growth 227
  | 2025-02-16 02:22:19 PST | RSS Growth 198
  | 2025-02-16 02:22:19 PST | RSS Growth 203
  | 2025-02-16 02:22:19 PST | RSS Growth 275
  | 2025-02-16 02:22:19 PST | RSS Growth 91
  | 2025-02-16 02:22:19 PST | RSS Growth 150
  | 2025-02-16 02:22:19 PST | RSS Growth 283
  | 2025-02-16 02:22:19 PST | RSS Growth 151
  | 2025-02-16 02:22:19 PST | RSS Growth 246
  | 2025-02-16 02:22:19 PST | RSS Growth 66
  | 2025-02-16 02:22:19 PST | RSS Growth 262
  | 2025-02-16 02:22:19 PST |  
  | 2025-02-16 02:22:19 PST | 156 \| lastRSS = rss;
  | 2025-02-16 02:22:19 PST | 157 \| }
  | 2025-02-16 02:22:19 PST | 158 \| Bun.gc(true);
  | 2025-02-16 02:22:19 PST | 159 \|
  | 2025-02-16 02:22:19 PST | 160 \| const rss = (process.memoryUsage.rss() / 1024 / 1024) \| 0;
  | 2025-02-16 02:22:19 PST | 161 \| expect(rss).toBeLessThan(4092);
  | 2025-02-16 02:22:19 PST | ^
  | 2025-02-16 02:22:19 PST | error: expect(received).toBeLessThan(expected)
  | 2025-02-16 02:22:19 PST |  
  | 2025-02-16 02:22:19 PST | Expected: < 4092
  | 2025-02-16 02:22:19 PST | Received: 4905
  | 2025-02-16 02:22:19 PST |  
  | 2025-02-16 02:22:19 PST | at (/var/lib/buildkite-agent/builds/ip-172-31-3-37/bun/bun/test/js/bun/http/bun-serve-static.test.ts:161:23)
  | 2025-02-16 02:22:19 PST | ✗ static > /big > stress (access .body) > arrayBuffer [5291.13ms]
  | 2025-02-16 02:22:19 PST | Start RSS 7207
[OOM killer kicks in]
```

https://buildkite.com/bun/bun/builds/11698#01950e27-d67a-40a6-b4ec-e381cd978a17

This particular test - `bun-serve-static.test.ts` makes many HTTP requests that respond with a large amount of data, repeatedly appending to a buffer internally (the `.body` getter forces it to stream internally). HTTP requests are processed in a dedicated thread with it's own mimalloc heap. The main thread receives chunks of the HTTP response from the HTTP thread and appends to an internal buffer (which uses `mi_malloc` instead of the heap-specific methods).

Note that the calls to `Bun.gc()` internally call `mi_collect(0)`

```ts
test.each(["arrayBuffer", "blob", "bytes", "text"])(
"%s",
async method => {
const byteSize = static_responses[path][method]?.size;

const bytes = method === "blob" ? static_responses[path] : await static_responses[path][method]();

// macOS limits backlog to 128.
// When we do the big request, reduce number of connections but increase number of iterations
const batchSize = Math.ceil((byteSize > 1024 * 1024 ? 48 : 64) / (isWindows ? 8 : 1));
const iterations = Math.ceil((byteSize > 1024 * 1024 ? 10 : 12) / (isWindows ? 8 : 1));

async function iterate() {
let array = new Array(batchSize);
const route = `${server.url}${path.substring(1)}`;
for (let i = 0; i < batchSize; i++) {
array[i] = fetch(route)
.then(res => {
expect(res.status).toBe(200);
expect(res.url).toBe(route);
if (label === "access .body") {
res.body;
}
return res[method]();
})
.then(output => {
expect(output).toStrictEqual(bytes);
});
}

await Promise.all(array);

Bun.gc();
}

for (let i = 0; i < iterations; i++) {
await iterate();
}

Bun.gc(true);
const baseline = (process.memoryUsage.rss() / 1024 / 1024) | 0;
let lastRSS = baseline;
console.log("Start RSS", baseline);
for (let i = 0; i < iterations; i++) {
await iterate();
const rss = (process.memoryUsage.rss() / 1024 / 1024) | 0;
if (lastRSS + 50 < rss) {
console.log("RSS Growth", rss - lastRSS);
}
lastRSS = rss;
}
Bun.gc(true);

const rss = (process.memoryUsage.rss() / 1024 / 1024) | 0;
expect(rss).toBeLessThan(4092);
const delta = rss - baseline;
console.log("Final RSS", rss);
console.log("Delta RSS", delta);
},
40 * 1000,
);
});
```

Start with Bun's test/bun/http/bun-serve-static.test.ts, especially the stress case around line 161, and compare its RSS behavior when using mimalloc v3 versus v2. The issue supplies a Buildkite failure and allocator/thread details but does not define a requested fix; done would require an agreed reproduction, diagnosis, and regression-free memory usage.

Written by the indexing model from the issue text.