Replace unsafe get_unchecked_mut with safe indexing in insert_score_depth and insert_score

[Copilot]

insert_score_depth and insert_score are public functions that previously used get_unchecked_mut without validating the caller-provided index, making them unsound — callers could trigger undefined behavior with an out-of-bounds index.

Changes Made

• Replaced unsafe { self.get_unchecked_mut(index) } with safe &mut self[index] indexing in both insert_score_depth and insert_score. This leverages the existing IndexMut implementation, which goes through DerefMut and slice indexing to automatically provide bounds checking and panic on invalid indices.
• Added #[should_panic] tests to verify that out-of-bounds access correctly panics for both functions.

This removes the unsafe blocks entirely while preserving the same public API and behavior for valid inputs.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Unsound problems in pleco_engine crate</issue_title>
<issue_description>Hi, thanks for your contribution to this great engine.

However, we have found some unsound problems in the crate pleco_engine.
For example, insert_score_depth and insert_score use get_unchecked_mut to access target RootMove without boundary checking for performance consideration.
These functions are actually unsafe since they ask the caller to guarantee the index is in-bound and the compiler fails to check this.

It is certainly ok to mark the unsafe functions as safe and use them with care in 'pleco'.
But considering the crate pleco_engine is available in crates.io and these functions are public, we think it's still necessary to make these functions more sound.

Suggestions

Here are some action choices we suggested:

1. Mark these functions as 'unsafe' which would lead to large code changes.
2. (recommended) Add assert! or at least debug_assert! macros to validate that the index is in-bound.</issue_description>

Comments on the Issue (you are @copilot in this section)

@chase-manning Thanks for the feedback! I would lean towards option 2. Please feel free to raise a PR for this 😃

• Fixes Unsound problems in pleco_engine crate #158

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[chase-manning]

@copilot linting ci broken

[Copilot]

@copilot linting ci broken

Fixed in c35a1d0 — the assert! macro lines were too long for cargo fmt. Reformatted them to multi-line style.

[chase-manning]

@copilot resolve merge conflicts

[Copilot]

@copilot resolve merge conflicts

Resolved in f46737c. The conflict was in insert_score_depth and insert_score — main had already changed these to use safe &mut self[index] indexing. Updated our branch to match, which also adds the out-of-bounds panic tests from main.