Require Documented Clippy Allows

[josecelano]

Parent EPIC: #2003

Goal: Require every newly introduced or modified Clippy allow attribute to carry a nearby specific rationale, with focused prospective enforcement that does not block on historical undocumented allowances.

Scope:

• Define rationale requirements for item and crate attributes.
• Require a removal condition or stable follow-up issue for temporary suppressions.
• Implement a focused validator with actionable diagnostics.
• Select a committed baseline or equivalent change-detection strategy that cannot silently accept new undocumented allows.

Boundary: Existing allowance remediation is tracked separately. Do not redesign the repository linter runner or CI architecture.

Verification: Run linter all, focused validator tests, and cargo test --doc --workspace.