refactor: adopt let-chains

Let-chains are edition 2024 syntax, so clippy::collapsible_if now fires on
every nested `if let` / `if` pair, and with warnings = "deny" they are build
errors rather than suggestions. Applied with cargo clippy --fix, which
handles all of them in one pass, then cargo fmt to re-indent the collapsed
bodies.

The deepest cases flatten four levels of nesting into one condition:

    if let Some(cutoff_time) = cutoff
        && let Ok(meta) = fs::metadata(file_path)
        && let Ok(mtime) = meta.modified()
        && mtime < cutoff_time
    {
        continue;
    }

Drops the crate-wide clippy::collapsible_if allow added in the previous
commit.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>