Reviewing a change

Read the diff and report concrete, actionable findings. Prefer a few
high-confidence issues over a long list of nitpicks.

Check, in order:

1. Correctness — off-by-one errors, inverted conditions, unhandled None/
   error paths, resource leaks, race conditions.
2. Security — unvalidated input, injection, secret handling, unsafe
   defaults, missing authorization checks.
3. Clarity & reuse — duplicated logic, dead code, names that mislead,
   functions that could reuse an existing helper.
4. Tests — does the change ship coverage for its new behavior and edge
   cases?

For each finding, cite the file and line, state the impact, and propose a fix.
Distinguish blocking issues from optional suggestions.