move daemon services to backend isolate

The merged UI/platform thread on macOS (Flutter 3.41) freezes on any
synchronous work — file I/O, Process.run, even isolate spawning during
early frames. No timing workaround (Timer, addPostFrameCallback,
Future.delayed) was reliable.

Fix: spawn a backend isolate that owns the DaemonDispatcher, GitClient,
PqlClient, FilesService, EditorRegistry, and Toolchain resolution. The
main isolate stays free for rendering. Communication uses SendPort with
the existing IPC message protocol (IpcRequest/IpcResponse/IpcEvent) —
zero new serialization.

New files:
  backend.dart       — spawns isolate, manages SendPort/ReceivePort
  backend_entry.dart — isolate entry point, boots all services
  isolate_client.dart — replaces InProcessClient for production

Toolchain now exposes resolveToolchainPaths() as a top-level function
with self-contained PATH expansion (no module-level state that would
prevent isolate message passing).

ClideTestApp gains boot-sequence tests: compute(), Isolate.run(),
sequential Process.run, and the full resolve+exec chain.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -29,6 +29,10 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Added
 
+- Backend isolate — all subprocess and file I/O runs in a dedicated
+  isolate, keeping the merged UI/platform thread on macOS free for
+  rendering. Communicates via SendPort using the existing IPC protocol.
+
 - Toolchain — centralized binary resolution replacing five ad-hoc
   mechanisms. Resolves git, pql, tmux, ptyc, shell once at boot via
   background isolate. Status bar reads `toolchain.missing` directly.