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>

test/daemon/pql_commands_errors_test.dart

@@ -8,7 +8,7 @@ library;
 import 'dart:io';
 
 import 'package:clide/clide.dart';
-import 'package:clide/kernel/src/toolchain.dart';
+import 'package:clide/kernel/src/toolchain_paths.dart';
 import 'package:clide/src/daemon/pql_commands.dart';
 import 'package:clide/src/pql/client.dart';
 import 'package:test/test.dart';
@@ -17,8 +17,7 @@ void main() {
   late DaemonDispatcher dispatcher;
 
   setUp(() {
-    final toolchain = Toolchain();
-    toolchain.applyResolved(const ResolvedPaths(pql: '/tmp/clide-no-such-pql-binary'));
+    final toolchain = ToolchainView.resolved(const ResolvedPaths(pql: '/tmp/clide-no-such-pql-binary'));
     final pql = PqlClient(workDir: Directory.current, toolchain: toolchain);
     dispatcher = DaemonDispatcher();
     registerPqlCommands(dispatcher, pql);