[deployment/web] Add --web-content-hash, Cache-Control, Overlay Deployment, and precache_manifest.json guidance for Flutter 3.50 #13825

Proposed Content

1. sites/docs/src/content/platform-integration/web/faq.md (https://docs.flutter.dev/platform-integration/web/faq)

• Replace L180-L181 ("Flutter doesn't currently support appending build IDs to resources automatically") with guidance on running flutter build web --web-content-hash, which automatically embeds 8-character truncated SHA-256 content hashes into compiled entrypoints (main.dart.<hash>.js, .wasm, .mjs), JS source maps (main.dart.<hash>.js.map, main.dart.<hash>.mjs.map when --source-maps is enabled), and 100% of files under build/web/assets/** (including AssetManifest.<hash>.bin, FontManifest.<hash>.json, NOTICES.<hash>, and shaders/**, with transparent runtime resolution in AssetManager and _flutter.buildConfig.extraAssets).
• Under ### How do I configure my cache headers? (L183-L236), add the firebase.json configuration for --web-content-hash builds (matching the CLI tip in packages/flutter_tools/lib/src/commands/build_web.dart) that pairs max-age=0, must-revalidate on bootstrap files with public, max-age=31536000, immutable on hashed entrypoints and assets/**:

{
  "hosting": {
    "public": "build/web",
    "ignore": ["**/*.map"],
    "headers": [
      {
        "source": "**",
        "headers": [
          {
            "key": "Cache-Control",
            "value": "max-age=0, must-revalidate"
          }
        ]
      },
      {
        "source": "assets/**",
        "headers": [
          {
            "key": "Cache-Control",
            "value": "public, max-age=31536000, immutable"
          }
        ]
      },
      {
        "source": "main.dart.*.+(js|wasm|mjs)",
        "headers": [
          {
            "key": "Cache-Control",
            "value": "public, max-age=31536000, immutable"
          }
        ]
      },
      {
        "source": "404.html",
        "headers": [
          {
            "key": "Cache-Control",
            "value": "max-age=0, must-revalidate"
          }
        ]
      }
    ],
    "rewrites": [
      {
        "source": "!(/assets/**|/canvaskit/**|/icons/**|/main.dart.*)",
        "destination": "/index.html"
      }
    ]
  }
}

2. sites/docs/src/content/deployment/web.md (https://docs.flutter.dev/deployment/web)

Add a Web content hashing (--web-content-hash) section (with a :::version-note for Flutter 3.50+) covering:

1. Hashed vs. Unhashed Build Outputs:
   • Immutable (public, max-age=31536000, immutable): main.dart.<hash>.js, main.dart.<hash>.wasm, main.dart.<hash>.mjs, main.dart.<hash>.js.map and main.dart.<hash>.mjs.map (if --source-maps is enabled and deployed), and all files under assets/** (including assets/AssetManifest.<hash>.bin, assets/AssetManifest.bin.<hash>.json, assets/FontManifest.<hash>.json, assets/NOTICES.<hash>, and assets/shaders/**).
   • Revalidate on Every Load (max-age=0, must-revalidate or no-cache): index.html, flutter_bootstrap.js, flutter.js, manifest.json, version.json, 404.html, canvaskit/** (when built with --no-web-resources-cdn), and main.dart.wasm.map (when built with --source-maps, since .wasm binaries embed an unhashed sourceMappingURL section).
2. Overlay Deployments (Zero-Downtime Rollouts for Active Tabs):
   • When deploying a new release N, an already-open browser tab running release N-1 may lazily fetch an image, font, or asset with release N-1's content hash.
   • To prevent mid-session 404 errors during rollout, retain at least the previous release's (N-1) hashed entrypoints and assets/** files alongside the new release's (N) hashed files in your CDN/storage bucket (or prune old hashes after a 24–72 hour grace window) while atomically updating index.html and flutter_bootstrap.js.
3. Custom Service Workers (build/web/precache_manifest.json):
   • When --web-content-hash is enabled, flutter build web generates build/web/precache_manifest.json ({"version": 1, "entries": [{"url": "...", "hash": "<8-char-sha256>", "size": ..., "urlHashed": true|false}]}) for consumption by custom PWA service workers or Workbox (where revision can be set to entry.urlHashed ? null : entry.hash).
4. Current Limitations:
   • --web-content-hash does not yet support deferred imports (deferred as) in either JavaScript (dart2js) or WebAssembly (dart2wasm / --enable-wasm-deferred-loading) builds (flutter/flutter#191917).

3. sites/docs/src/content/platform-integration/web/initialization.md (https://docs.flutter.dev/platform-integration/web/initialization)

• Note under ## Write a custom bootstrap script (L83-L103) that --web-content-hash injects the hashed entrypoint filenames (mainJsPath, jsSupportRuntimePath, mainWasmPath) and extraAssets (assetManifest, fontManifest, AssetManifest.bin, NOTICES, shaders/**) into {{flutter_build_config}}. Custom web/index.html or web/flutter_bootstrap.js files must use {{flutter_build_config}} with _flutter.loader.load() rather than hardcoding "main.dart.js" or calling the deprecated loadEntrypoint API.