perf: Phase 1 critical performance fixes + production macOS build

Performance:
- Move JSON parsing to background isolate via Isolate.run() (eliminates 2-4s UI freeze)
- Cache filteredEntries with key-based invalidation (eliminates O(n) recomputation)
- Debounce search queries at 300ms (prevents cascade rebuilds on keystroke)
- Flatten timeline from 2-level turn×response Column to single virtualized ListView.builder
- Add RepaintBoundary per timeline item (isolates repaints during scroll)
- Use context.select for granular rebuilds instead of top-level context.watch
- Lazy ExpandableCard: child not built until first expand (replaces AnimatedCrossFade)
- Use IndexedStack in AppShell (preserves screen state across tab switches)

Fixes:
- Collect parse errors instead of silently swallowing them
- Show parse error count in timeline filter bar
- Fix overflow in tokens screen pie chart legend

Build:
- Configure Developer ID signing with hardened runtime for production distribution
- Enable secure timestamps for notarization
- Update app name to Claude Session Viewer
- Signed, notarized, stapled DMG distribution

lib/providers/session_provider.dart

@@ -1,3 +1,5 @@
+import 'dart:async';
+
 import 'package:flutter/foundation.dart';
 import '../models/agent_info.dart';
 import '../models/log_entry.dart';
@@ -10,12 +12,21 @@ class SessionProvider with ChangeNotifier {
   bool _isLoading = false;
   String? _error;
   String? _filePath;
+  List<ParseError> _parseErrors = const [];
 
   // Filter state
   String? _selectedAgentId;
   Set<String> _visibleTypes = {'user', 'assistant', 'system'};
   String _searchQuery = '';
 
+  // Cached filtered results
+  List<LogEntry> _cachedFilteredEntries = const [];
+  // Cache key — tracks what was used to compute the cache
+  _FilterKey? _cachedFilterKey;
+
+  // Debounce timer for search
+  Timer? _searchDebounce;
+
   SessionLog? get session => _session;
   bool get isLoading => _isLoading;
   String? get error => _error;
@@ -23,15 +34,30 @@ class SessionProvider with ChangeNotifier {
   String? get selectedAgentId => _selectedAgentId;
   Set<String> get visibleTypes => _visibleTypes;
   String get searchQuery => _searchQuery;
+  List<ParseError> get parseErrors => _parseErrors;
 
   AgentInfo? get selectedAgent {
     if (_session == null || _selectedAgentId == null) return null;
-    return _session!.agents
-        .where((a) => a.id == _selectedAgentId)
-        .firstOrNull;
+    return _session!.agents.where((a) => a.id == _selectedAgentId).firstOrNull;
   }
 
   List<LogEntry> get filteredEntries {
+    final key = _FilterKey(
+      selectedAgentId: _selectedAgentId,
+      visibleTypes: _visibleTypes,
+      searchQuery: _searchQuery,
+      hasSession: _session != null,
+    );
+
+    if (_cachedFilterKey == key) return _cachedFilteredEntries;
+
+    // Recompute
+    _cachedFilterKey = key;
+    _cachedFilteredEntries = _computeFilteredEntries();
+    return _cachedFilteredEntries;
+  }
+
+  List<LogEntry> _computeFilteredEntries() {
     if (_session == null) return [];
 
     List<LogEntry> entries;
@@ -73,10 +99,14 @@ class SessionProvider with ChangeNotifier {
     _filePath = filePath;
     _selectedAgentId = null;
     _searchQuery = '';
+    _parseErrors = const [];
+    _invalidateCache();
     notifyListeners();
 
     try {
-      _session = await _parser.parseFile(filePath);
+      final result = await _parser.parseFile(filePath);
+      _session = result.sessionLog;
+      _parseErrors = result.errors;
       _error = null;
     } catch (e) {
       _error = e.toString();
@@ -89,6 +119,7 @@ class SessionProvider with ChangeNotifier {
 
   void selectAgent(String? agentId) {
     _selectedAgentId = agentId;
+    _invalidateCache();
     notifyListeners();
   }
 
@@ -98,12 +129,18 @@ class SessionProvider with ChangeNotifier {
     } else {
       _visibleTypes = Set.from(_visibleTypes)..add(type);
     }
+    _invalidateCache();
     notifyListeners();
   }
 
   void setSearchQuery(String query) {
     _searchQuery = query;
-    notifyListeners();
+    // Update cache immediately but debounce notifyListeners
+    _invalidateCache();
+    _searchDebounce?.cancel();
+    _searchDebounce = Timer(const Duration(milliseconds: 300), () {
+      notifyListeners();
+    });
   }
 
   void clearSession() {
@@ -112,6 +149,55 @@ class SessionProvider with ChangeNotifier {
     _error = null;
     _selectedAgentId = null;
     _searchQuery = '';
+    _parseErrors = const [];
+    _searchDebounce?.cancel();
+    _invalidateCache();
     notifyListeners();
   }
+
+  void _invalidateCache() {
+    _cachedFilterKey = null;
+  }
+
+  @override
+  void dispose() {
+    _searchDebounce?.cancel();
+    super.dispose();
+  }
+}
+
+/// Immutable key to detect whether the filter result has changed.
+class _FilterKey {
+  final String? selectedAgentId;
+  final Set<String> visibleTypes;
+  final String searchQuery;
+  final bool hasSession;
+
+  const _FilterKey({
+    required this.selectedAgentId,
+    required this.visibleTypes,
+    required this.searchQuery,
+    required this.hasSession,
+  });
+
+  @override
+  bool operator ==(Object other) =>
+      identical(this, other) ||
+      other is _FilterKey &&
+          selectedAgentId == other.selectedAgentId &&
+          _setEquals(visibleTypes, other.visibleTypes) &&
+          searchQuery == other.searchQuery &&
+          hasSession == other.hasSession;
+
+  @override
+  int get hashCode =>
+      Object.hash(selectedAgentId, Object.hashAll(visibleTypes), searchQuery, hasSession);
+
+  static bool _setEquals<T>(Set<T> a, Set<T> b) {
+    if (a.length != b.length) return false;
+    for (final v in a) {
+      if (!b.contains(v)) return false;
+    }
+    return true;
+  }
 }