keep the clide.dart barrel Flutter-free

`lib/kernel/src/toolchain.dart` is a `ChangeNotifier`, so it pulls in
`package:flutter/foundation.dart`. `GitClient` and `PqlClient` imported
it for the resolved binary paths, which leaked Flutter through the
`package:clide/clide.dart` barrel — breaking `dart test` on every core
subsystem suite (`ci/test_core.sh`), since pure Dart can't compile
Flutter packages.

Split the Flutter-free pieces into `toolchain_paths.dart`: `ResolvedPaths`,
`resolveToolchainPaths`, and a new read-only `ToolchainView` interface
with a `ToolchainView.resolved()` const factory. `Toolchain` now
implements `ToolchainView`; the clients depend on the interface. Core
test setups that built a `Toolchain` just to call `applyResolved`
switch to the factory.

Co-Authored-By: Claude <noreply@anthropic.com>

lib/src/git/client.dart

@@ -1,4 +1,4 @@
-/// Typed git client backed by [Toolchain].
+/// Typed git client backed by a [ToolchainView].
 ///
 /// Every subprocess call goes through [_run] which uses the resolved
 /// absolute binary path from the toolchain. Parsing is delegated to
@@ -7,7 +7,7 @@ library;
 
 import 'dart:io';
 
-import '../../kernel/src/toolchain.dart';
+import '../../kernel/src/toolchain_paths.dart';
 import 'diff.dart' show GitDiff, parseDiffOutput;
 import 'operations.dart' show GitException, GitLogEntry;
 import 'status.dart';
@@ -15,7 +15,7 @@ import 'status.dart';
 class GitClient {
   GitClient({required this.toolchain, required this.workDir});
 
-  final Toolchain toolchain;
+  final ToolchainView toolchain;
   final Directory workDir;
 
   // -- queries --------------------------------------------------------------