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/pql/client_test.dart

@@ -5,15 +5,14 @@ library;
 
 import 'dart:io';
 
-import 'package:clide/kernel/src/toolchain.dart';
+import 'package:clide/kernel/src/toolchain_paths.dart';
 import 'package:clide/src/pql/client.dart';
 import 'package:test/test.dart';
 
 void main() {
   late PqlClient pql;
   setUp(() {
-    final toolchain = Toolchain();
-    toolchain.applyResolved(Toolchain.resolvePaths(workspaceRoot: Directory.current.path));
+    final toolchain = ToolchainView.resolved(resolveToolchainPaths(Directory.current.path));
     pql = PqlClient(workDir: Directory.current, toolchain: toolchain);
   });
 
@@ -123,9 +122,8 @@ void main() {
 
   group('PqlClient — error surface', () {
     test('non-existent pql binary raises a PqlException with ProcessException details', () async {
-      final t = Toolchain();
       // Inject a bad path — Process.run will throw ProcessException.
-      t.applyResolved(const ResolvedPaths(pql: '/tmp/clide-no-such-pql-binary'));
+      final t = ToolchainView.resolved(const ResolvedPaths(pql: '/tmp/clide-no-such-pql-binary'));
       final bad = PqlClient(workDir: Directory.current, toolchain: t);
       try {
         await bad.files();