[Bug] Static fileIoExecutor causes thread leak and StrictMode violations #154

Description
The BlockCanary class initializes a private static final Executor named fileIoExecutor for handling log zipping and writing. However, this executor is never shut down, even when BlockCanary.stop() is invoked.

Root Cause

1. Static Resource: The executor is defined as private static final at line 133.

2. Missing Lifecycle Management: There is no mechanism to terminate the fileIoExecutor. The stop() method (Line 103) only stops the CPU and Stack samplers but leaves the IO thread pool active.

3. Process-Level Scope: In Android, this static thread persists as long as the application process is alive, which is broader than the lifecycle of the monitoring session.

Impact

1. Thread Leak: The thread named "File-IO" persists indefinitely, leaking resources.

2. StrictMode Violations: This causes StrictMode thread leak warnings (e.g., Detecting explicit GC) during integration testing or when the app attempts to exit cleanly.

Relevant Code
File: com/github/moduth/blockcanary/BlockCanary.java
// Line 133
private static final Executor fileIoExecutor = newSingleThreadExecutor("File-IO");

// Line 103: stop() does not handle the executor
public void stop() {
if (mMonitorStarted) {
// ...
mBlockCanaryCore.stackSampler.stop();
mBlockCanaryCore.cpuSampler.stop();
// fileIoExecutor is left running
}
}

Suggested Fix
Modify the lifecycle of fileIoExecutor so it can be shut down inside the stop() method, or provide an explicit release() API to terminate the underlying thread pool.