[Feature]: Add integration tests for Tracer profiling engine

## Summary

Add integration tests that exercise the core tracing engine (`Tracer._trace`, `get_trace_data`, `show_results`) against real Python code, not just `_is_user_code` path filtering.

## Problem

`tests/test_tracer.py` covers 12 scenarios for `_is_user_code()` (stdlib, site-packages, venv markers, symlinks, etc.) but does not verify that profiling actually records function calls, timings, callee relationships, or call-tree output. A regression in `_trace()` or `get_trace_data()` would go undetected.

## Proposed Solution

Add a new test module (e.g. `tests/test_tracer_integration.py`) with small fixture scripts that:

1. Start/stop a `Tracer` around `runpy.run_path()` or an inline function call
2. Assert `get_trace_data()` returns expected function keys, call counts, and total times
3. Cover `--ignore` pattern filtering via `_get_key()` / `_is_ignored()`
4. Cover exception stack unwinding (the fallback path in `_trace()` when return frame doesn't match stack top)
5. Optionally test the public API (`start_trace`, `stop_trace`, `show_results`) exported from `oracletrace/__init__.py`

## Use Case

The tracer is the core of OracleTrace. Integration tests give confidence that profiling, aggregation, and filtering work end-to-end — especially after changes to `_trace()` or user-code detection.

## Example (optional)

```python
# Fixture script: calls nested functions with known structure
def outer():
inner()

def inner():
pass

outer()
```

```text
# Expected trace keys like "script.py:outer", "script.py:inner"
# Expected call_count and callee relationships
```

## Alternatives Considered

- Extend `tests/test_tracer.py` in place — possible, but separation keeps unit vs integration tests clear.
- Only add CLI-level tests — CLI tests mock `Tracer` and would not catch engine regressions.

## Additional Context

Currently untested in `oracletrace/tracer.py`:
- `_trace()` call/return handling and exception unwind (lines 174–215)
- `get_trace_data()` (lines 271–296)
- `show_results()` table and logic-flow tree (lines 217–269)
- Public helpers `start_trace` / `stop_trace` (lines 302–322)

## Checklist

- [x] I searched existing issues before opening this request
- [x] I described the problem and why this feature is useful
- [x] I provided enough detail for implementation discussion