build: target Android SDK 37, use Rust 1.98.1 and add a safe mode run

Moves to AGP 9.4.1 and Gradle 9.8.0; Rust 1.98.1 lets macOS 27 load
rust_api. Android 17 treats the tun subnet as local network, so starting
the VPN asks for that permission and falls back to gvisor without it.
Signed release builds upload native symbols to Crashlytics.

.agents/flutter-upgrade.md

@@ -0,0 +1,31 @@
+# Flutter Upgrade Checklist
+
+Read this before changing `FLUTTER_VERSION` in `.github/workflows/build.yaml` or `.fvmrc`. Each entry is a
+workaround tied to a Flutter bug: what it does, where it lives, and what has to be true before it can go. Remove an
+entry together with its workaround; add one whenever a new workaround is pinned to the SDK.
+
+## Stable cadence
+
+Flutter ships a stable release about once a quarter, three minor versions apart (3.41 in February, 3.44 in May, 3.47
+in August 2026). The two minors in between (3.48, 3.49) are beta only, so a fix that lands on `main` reaches stable
+in the next multiple, 3.50, expected around mid-November 2026. Hotfix releases (3.47.x) carry only cherry-picks
+listed in the SDK's `CHANGELOG.md`.
+
+## Impeller disabled on Intel macOS release builds
+
+- **Bug:** [flutter/flutter#191538](https://github.com/flutter/flutter/issues/191538). From 3.47, when Impeller
+  became the default desktop renderer, Intel (x86_64) Macs flicker across the whole window while scrolling or
+  animating, and text can render garbled. Apple Silicon is not affected.
+- **Fix upstream:** landed on `main` before 2026-08-25 and verified on `3.48.0-1.0.pre-404`; it is not in any
+  3.47.x hotfix (checked through 3.47.5).
+- **Workaround:** the release job for `macos-15-intel` (`arch: amd64`) runs
+  `plutil -replace FLTEnableImpeller -bool NO macos/Runner/Info.plist` before `dart setup.dart`, so only the Intel
+  DMG falls back to Skia. The checked-in `Info.plist` does not set the key, so local builds and the arm64 DMG keep
+  Impeller. The macOS embedder reads the key at launch in `FlutterDartProject.mm` (`enableImpeller`), release
+  builds included.
+- **Side effect:** Skia on Intel renders blur, shadows and text slightly differently; judge Intel-only visual reports
+  against Skia, not Impeller.
+- **Remove when:** `FLUTTER_VERSION` moves to a stable that contains the fix (3.50 or later, or a 3.47.x whose
+  changelog lists #191538). Before dropping the step, build the Intel DMG without it and scroll the proxies and
+  dashboard pages on an Intel Mac with its integrated GPU driving the built-in display; an external display can
+  switch to the discrete GPU and hide the flicker.