Enable clippy::todo lint #88

Summary

Enable the clippy::todo lint (deny level) to prevent unfinished todo!() placeholder macros from being merged into production code.

What it catches

clippy::todo flags any use of the todo!() macro. todo!() is meant as a temporary, compile-time-OK placeholder during development — it panics at runtime ("not yet implemented"). When one accidentally survives review and ships, an end user triggers a hard panic instead of real behavior.

Why it improves code quality

• A leftover todo!() is a latent runtime panic — the compiler accepts it, so nothing catches it without this lint.
• It is exactly the same class of "leftover dev artifact" the crate already guards against with dbg_macro = "deny". This extends that intent.
• Cost is zero today: the crate currently has no todo!() usages, so enabling this at deny introduces no violations and no code churn. It is purely a guardrail against future regressions.

It belongs to Clippy's opt-in restriction group, so it is not covered by the pedantic / nursery / cargo groups already enabled in Cargo.toml.

Proposed change

Add to the existing [lints.clippy] table in Cargo.toml, beside dbg_macro:

# Forbid leftover `todo!()` placeholder macros from reaching production
todo = "deny"

A PR implementing this follows.

This issue was opened by the "Clippy lint improvements → issue + PR (Rust repos) → Slack" routine of moadim.