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/git_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/git_commands.dart';
 import 'package:test/test.dart';
 
@@ -18,8 +18,7 @@ void main() {
 
   setUp(() async {
     sandbox = await Directory.systemTemp.createTemp('clide-git-cmd-err-');
-    final toolchain = Toolchain();
-    toolchain.applyResolved(const ResolvedPaths(git: '/tmp/clide-no-such-git-binary'));
+    final toolchain = ToolchainView.resolved(const ResolvedPaths(git: '/tmp/clide-no-such-git-binary'));
     final git = GitClient(toolchain: toolchain, workDir: sandbox);
     dispatcher = DaemonDispatcher();
     final sink = RecordingEventSink();

test/daemon/git_commands_test.dart

@@ -1,7 +1,7 @@
 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/git_commands.dart';
 import 'package:test/test.dart';
 
@@ -33,8 +33,7 @@ void main() {
 
     sink = RecordingEventSink();
     dispatcher = DaemonDispatcher();
-    final toolchain = Toolchain();
-    toolchain.applyResolved(Toolchain.resolvePaths(workspaceRoot: sandbox.path));
+    final toolchain = ToolchainView.resolved(resolveToolchainPaths(sandbox.path));
     final gitClient = GitClient(toolchain: toolchain, workDir: sandbox);
     registerGitCommands(dispatcher, gitClient, sink);
   });

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);

test/daemon/pql_commands_test.dart

@@ -1,7 +1,7 @@
 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:test/test.dart';
 
@@ -10,8 +10,7 @@ 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);
     dispatcher = DaemonDispatcher();
     registerPqlCommands(dispatcher, pql);

test/git/client_test.dart

@@ -5,16 +5,12 @@ library;
 
 import 'dart:io';
 
-import 'package:clide/kernel/src/toolchain.dart';
+import 'package:clide/kernel/src/toolchain_paths.dart';
 import 'package:clide/src/git/client.dart';
 import 'package:clide/src/git/operations.dart' show GitException;
 import 'package:test/test.dart';
 
-Toolchain _toolchain() {
-  final t = Toolchain();
-  t.applyResolved(Toolchain.resolvePaths(workspaceRoot: Directory.current.path));
-  return t;
-}
+ToolchainView _toolchain() => ToolchainView.resolved(resolveToolchainPaths(Directory.current.path));
 
 Future<Directory> _newRepo({String filename = 'file.txt', String contents = 'hello\n'}) async {
   final dir = await Directory.systemTemp.createTemp('clide-git-client-');
@@ -195,8 +191,7 @@ void main() {
 
   group('GitClient — error surface', () {
     test('a bad git binary path makes _run throw GitException', () async {
-      final t = Toolchain();
-      t.applyResolved(const ResolvedPaths(git: '/tmp/clide-no-such-git-binary'));
+      final t = ToolchainView.resolved(const ResolvedPaths(git: '/tmp/clide-no-such-git-binary'));
       final dir = await Directory.systemTemp.createTemp('clide-git-bad-');
       addTearDown(() => dir.deleteSync(recursive: true));
       final git = GitClient(toolchain: t, workDir: dir);

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();