docs: explain cast trait and vault double-spend checks

[HkSolDev]

Add documentation comments to the Cast trait definition explaining its safety mechanism and track_caller debugging logic.

Also add inline comments inside the withdraw instruction handler explaining the reload and ledger double-spend validation checks.

Summary by CodeRabbit

Documentation

• Added clarification documentation for vault token balance verification procedures.
• Improved documentation for safe type casting behavior and error handling.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 2889a803-945b-4378-bec6-bcf799f7b537

📥 Commits

Reviewing files that changed from the base of the PR and between 0ae3e3b and 3b8c7a7.

⛔ Files ignored due to path filters (1)

• Cargo.lock is excluded by !**/*.lock

📒 Files selected for processing (2)

• programs/drift/src/instructions/user.rs
• programs/drift/src/math/casting.rs

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Walkthrough

Two sets of documentation comments are added with no logic changes: nine lines of Rust doc comments on the Cast helper trait in casting.rs describing safe conversion behavior, and two inline comments in handle_withdraw in user.rs explaining the vault-vs-ledger balance check.

Changes

Inline Documentation Comments

Layer / File(s)	Summary
Cast trait and handle_withdraw docs programs/drift/src/math/casting.rs, programs/drift/src/instructions/user.rs	Adds doc comments to the Cast trait describing try_into() usage, #[track_caller] behavior, and CastingFailure error semantics; adds two inline comments in handle_withdraw describing the post-withdraw vault token balance check against the internal ledger and its insolvency/double-spend meaning.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Poem

A rabbit scribbled notes with care,
"This vault check matters — beware!"
The Cast trait got its story told,
Safe conversions, tracked and bold.
🐇 Good docs make the warren wise!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main changes: adding documentation to explain the Cast trait and vault double-spend validation checks.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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