Enable clippy::manual_let_else lint

[tupe12334]

Summary

Enable the Clippy lint clippy::manual_let_else at deny, continuing the incremental clippy ratchet documented in core/Cargo.toml ([lints.clippy]).

What it catches

A diverging match or if let whose only purpose is to bind a value (and otherwise return/break/continue/panic) — e.g.

let value = match opt {
    Some(v) => v,
    None => return,
};

It flags those and suggests the dedicated let ... else { ... } form:

let Some(value) = opt else { return };

Why it improves code quality

• Less boilerplate, clearer happy path — the binding and its divergence are stated once instead of a multi-line match that buries the main flow.
• Scope clarity — let ... else keeps the bound name in the enclosing scope by construction; the manual form is easy to get subtly wrong.
• Consistency — pairs with the existing option_if_let_else deny and the project's broader "prefer the dedicated control-flow form" stance.

Scope

• One rule only: manual_let_else = "deny" in core/Cargo.toml.
• The codebase currently has zero violations, so this lands clean as a guard against future regressions — cargo clippy --all-targets and cargo test both pass.

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This issue was opened by the Add lint rule → issue + PR (owned repos + my orgs) → Slack routine of moadim. cc @tupe12334