chore(lint): enable clippy::panic_in_result_fn

[tupe12334]

Summary

Enable the clippy::panic_in_result_fn restriction lint, which flags panic!, todo!, unimplemented!, and unreachable! calls inside functions that return Result or Option.

Why

When a function's return type already models fallibility, an internal panic bypasses structured error propagation — the process crashes instead of returning Err(_)/None for the caller to handle. This lint enforces the invariant that Result/Option-returning functions stay within the type system's error-handling contract.

What changed

Added to [lints.clippy] in Cargo.toml:

# Prevent panic!/todo!/unimplemented!/unreachable! inside Result/Option-returning
# functions. When a function already signals fallibility via its return type,
# an internal panic bypasses structured error propagation and crashes the
# process instead of returning Err(_)/None for the caller to handle.
panic_in_result_fn = "deny"

The codebase has zero violations today — this commit locks in that clean state and prevents future regressions.

Verification

cargo clippy --all-targets passes with no warnings or errors. Pre-push hook (clippy + build + 100% coverage) also passed.

Closes #95

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This pull request was opened by the Clippy lint improvements → issue + PR (Rust repos) → Slack routine of moadim. CC @tupe12334

[tupe12334]

Resolved merge conflict against main via manual-merge-both-kept (this PR's panic_in_result_fn lint had already landed via a sibling PR, so the merge keeps main's version alongside the expect_used lint from another sibling — no functional change).

Posted by the moadim routine "Resolve conflicts on code-quality PRs (non-guiddeco)".