fix(flutter): fetch the arm64 macOS archive on Apple Silicon hosts

[TeddyHarnat]

Problem

The macOS SDK repository always downloads the x64 release archive
(flutter_macos_<version>-stable.zip). Its Dart binaries are x86_64, so on an
Apple Silicon host without Rosetta 2 every fetch fails during the warm-up steps
with:

flutter/bin/cache/dart-sdk/bin/dart: Bad CPU type in executable

making the toolchain unusable on such machines (Rosetta 2 is not installed by
default, and installing it requires admin rights).

Fix

Select the archive variant by host architecture in _flutter_repo_impl: on
aarch64/arm64 macOS the repository fetches
flutter_macos_arm64_<version>-stable.zip. The URL's directory component stays
macos; only the file name carries the variant, matching how Flutter publishes
its releases.

Integrity is resolved against the variant actually fetched:

• scripts/update_flutter_versions.sh now extracts both macOS hashes from the
  release metadata, and the regenerated versions.bzl records a
  macos_arm64 SRI for every version that publishes one. (The refresh also
  picked up the stable releases published since the last run, 3.44.6–3.47.1.)
• The flutter.toolchain(integrity = {...}) escape hatch accepts a
  "macos_arm64" key, carried into the repository rule by a new
  integrity_arm64 attribute — backward compatible with existing callers.

Docs (docs/extensions.md via //docs:update) and CHANGELOG updated; the
lockfile changes are the resolved extension's new attribute and bzl digest.

Verification

On an arm64 Mac without Rosetta 2 (the previously broken scenario):

• bazel test //... — 14/14 pass
• examples/hello_world: bazel test --override_module=rules_flutter=<tree> //...
  builds and the widget test passes; the fetched dart-sdk binaries are
  Mach-O 64-bit executable arm64
• The generated 3.38.4 macos_arm64 SRI matches a hash computed independently
  from a separately downloaded copy of the archive
• buildifier check clean

🤖 Generated with Claude Code