```python
# /// intent
# goal   = "Count characters in each .cpp file under src and write report.txt"
# reads  = ["src/*.cpp"]
# writes = ["report.txt"]
# output = "none"
# ///

from pathlib import Path


def char_count(path):
    text = path.read_text()
    return len(text)


src_path = Path("src")
paths = list(src_path.glob("*.cpp"))
paths = sorted(paths, key=lambda p: p.name)

lines = []
for p in paths:
    n = char_count(p)
    lines.append(f"{p.name}: {n}")

result = "\n".join(lines) + "\n"
Path("report.txt").write_text(result)
```