feat: custom-checker

[xxori]

Add custom checker python support for aucpl problem test. It will check for checker.py files next to problem.md, and execute them instead of just doing string comparison if they exist. Here is an example of a valid checker.py:

def check(process_output, judge_output):
    return abs(float(process_output) - float(judge_output)) <= 0.5

Limitation: Often you will import dmoj things in the checkers, which doesn't work here. The functions can only be raw Python scripts. Also, you can only use the process output and judge output, although DMOJ passes in much more information to their custom judges. https://docs.dmoj.ca/#/problem_format/custom_checkers?id=custom-checkers-1

[Copilot]

Pull request overview

Adds support for DMOJ-style custom Python checkers to aucpl problem test, allowing problems to define a checker.py that determines pass/fail instead of strict output byte comparison.

Changes:

• Add embedded Python runner script and Rust glue code to execute checker.py per test case and parse a boolean result.
• Detect checker.py in the problem directory and switch the test verdict mechanism accordingly.
• Document checker.py usage and expected function signature in README.md.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
crates/cli/src/problem/test.rs	Adds checker.py discovery and per-test execution via python3, wiring outputs into a check() call.
README.md	Documents custom checker behavior and shows folder structure including checker.py.

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