ci(windows): use DotSlash for zstd in rust-release-windows (#11542)

## Why
Installing `zstd` via Chocolatey in
`.github/workflows/rust-release-windows.yml` has been taking about a
minute on Windows release runs. This adds avoidable latency to each
release job.

Using DotSlash removes that package-manager install step and pins the
exact binary we use for compression.

## What Changed
- Added `.github/workflows/zstd`, a DotSlash wrapper that fetches
`zstd-v1.5.7-win64.zip` with pinned size and digest.
- Updated `.github/workflows/rust-release-windows.yml` to:
  - install DotSlash via `facebook/install-dotslash@v2`
- replace `zstd -T0 -19 ...` with
`${GITHUB_WORKSPACE}/.github/workflows/zstd -T0 -19 ...`
- `windows-aarch64` uses the same win64 upstream zstd artifact because
upstream releases currently publish `win32` and `win64` binaries.

## Verification
- Verified the workflow now resolves the DotSlash file from
`${GITHUB_WORKSPACE}` while the job runs with `working-directory:
codex-rs`.
- Ran VS Code diagnostics on changed files:
  - `.github/workflows/rust-release-windows.yml`
  - `.github/workflows/zstd`

.github/workflows/zstd

@@ -0,0 +1,46 @@
+#!/usr/bin/env dotslash
+
+// This DotSlash file wraps zstd for Windows runners.
+// The upstream release provides win32/win64 binaries; for windows-aarch64 we
+// use the win64 artifact via Windows x64 emulation.
+{
+  "name": "zstd",
+  "platforms": {
+    "windows-x86_64": {
+      "size": 1747181,
+      "hash": "sha256",
+      "digest": "acb4e8111511749dc7a3ebedca9b04190e37a17afeb73f55d4425dbf0b90fad9",
+      "format": "zip",
+      "path": "zstd-v1.5.7-win64/zstd.exe",
+      "providers": [
+        {
+          "url": "https://github.com/facebook/zstd/releases/download/v1.5.7/zstd-v1.5.7-win64.zip"
+        },
+        {
+          "type": "github-release",
+          "repo": "facebook/zstd",
+          "tag": "v1.5.7",
+          "name": "zstd-v1.5.7-win64.zip"
+        }
+      ]
+    },
+    "windows-aarch64": {
+      "size": 1747181,
+      "hash": "sha256",
+      "digest": "acb4e8111511749dc7a3ebedca9b04190e37a17afeb73f55d4425dbf0b90fad9",
+      "format": "zip",
+      "path": "zstd-v1.5.7-win64/zstd.exe",
+      "providers": [
+        {
+          "url": "https://github.com/facebook/zstd/releases/download/v1.5.7/zstd-v1.5.7-win64.zip"
+        },
+        {
+          "type": "github-release",
+          "repo": "facebook/zstd",
+          "tag": "v1.5.7",
+          "name": "zstd-v1.5.7-win64.zip"
+        }
+      ]
+    }
+  }
+}