docs(ios): document Xcode 27 development setup (#8400)

Document Xcode 27.0 as the recommended iOS development toolchain while
retaining Xcode 26 compatibility. Explain per-command selection, the
known Flutter native-assets compiler/SDK mismatch on hosts with multiple
Xcode versions, and where CI and release toolchains are configured.

The compatibility changes are already merged in
[#8379](https://github.com/block/buzz/pull/8379) and
[#8398](https://github.com/block/buzz/pull/8398). Related open work is
[#8330](https://github.com/block/buzz/pull/8330); this PR changes
documentation only. The release version bump remains separate in
[buzz-releases#110](https://github.com/squareup/buzz-releases/pull/110).

### Validation

- Ordinary unsigned Release build passed on [main at
73962435b](73962435b6)
with exact Xcode 27.0 (27A266a), the pinned Flutter SDK, and normal
CocoaPods installation. Buzz.app contains NotificationService.appex. No
dependency patches or plugin-validation bypasses were used.
- The merged direct-C change passed native tests on Xcode 26.5 and 27.0,
full local CI, and hosted Native and unsigned Release validation. This
documentation-only follow-up preserves that code.
- Documentation diff checks and local pre-push checks passed.

---------

Signed-off-by: Tom Brow <tomb@block.xyz>

CONTRIBUTING.md

@@ -84,6 +84,7 @@ We review as capacity allows — focused PRs that follow this guide move fastest
 | Node.js | 24+ | Required for desktop app commands and `just ci` |
 | pnpm | 10+ | Required for desktop app commands and `just ci` |
 | Flutter | 3.41+ | Required for mobile app — install via [flutter.dev](https://docs.flutter.dev/get-started/install) |
+| Xcode | 27.0 recommended; 26 supported | Required for iOS builds; see [iOS toolchain setup](mobile/README.md#ios-toolchain) |
 | Docker | 24+ | For Postgres, Redis, MinIO |
 | `just` | latest | Task runner — `cargo install just` |
 | `lefthook` | 2.1.3 (Hermit-pinned) | Auto-installed by `just hooks` — no manual install needed |

mobile/README.md

@@ -21,6 +21,34 @@ external Flutter SDK has touched `mobile/.dart_tool`, rerun `mobile-install`
 with the pinned SDK before building so `flutter_test`, `sky_engine`, and the
 engine all come from the same Flutter version.
 
+### iOS toolchain
+
+Use Xcode 27.0 for iOS development and build validation on an Apple silicon
+Mac running macOS Tahoe 26.6 or later, as required by the
+[Xcode 27 release notes](https://developer.apple.com/documentation/xcode-release-notes/xcode-27-release-notes).
+On Intel Macs or hosts that cannot meet those requirements, use a compatible
+Xcode 26 version, which remains supported. Activate the repository's pinned
+Flutter SDK with Hermit as described above.
+
+When multiple Xcode versions are installed, select the intended version for
+the command with `DEVELOPER_DIR`, for example:
+
+```bash
+DEVELOPER_DIR=/Applications/Xcode-27.0.0.app/Contents/Developer xcodebuild -version
+```
+
+Adjust the path to your installation. Keep the compiler and SDK from the same
+Xcode installation. The pinned Flutter native-assets tooling can discover the
+globally selected SDK even when `DEVELOPER_DIR` selects another compiler, so
+use a build machine with a matching default Xcode when validating either
+Xcode 26 or Xcode 27.
+
+The native cryptography dependency is pinned upstream C source in
+`ios/BuzzPushKit/Sources/CSecp256k1`; it requires no Swift build-plugin approval.
+GitHub's mobile CI currently uses the Xcode default of its `macos-latest`
+runner. The internal release toolchain is configured separately in
+[buzz-releases](https://github.com/squareup/buzz-releases).
+
 ## Run
 
 ```bash