fix(cli): replace unhelpful panic message with an actionable one

[soy-chrislo]

While reviewing rumdl's error handling I noticed the panic handler in main.rs always prints Uncaught panic: Any { .. } regardless of what actually caused the crash. That happens because catch_unwind returns the panic payload as Box<dyn Any + Send>, and printing it with {:?} shows the type wrapper, not the message.

I first tried extracting the message with downcast_ref::<&str>() / downcast_ref::<String>(), the pattern shown in the stdlib docs for PanicHookInfo::payload(). That turned out to be redundant: Rust's default panic hook already prints the real message, file and line before catch_unwind gets a chance to run, so the extracted text would just repeat the line already shown above it.

Instead I replaced the message with something that adds information the default hook does not provide: the rumdl version and a link to file a bug, since an uncaught panic here is always a bug.

Before:

[rumdl panic handler] Uncaught panic: Any { .. }

After (forced a panic locally to verify, reverted before this commit):

thread 'main' (330372) panicked at src/main.rs:315:9:
...
[rumdl panic handler] rumdl crashed unexpectedly (v0.2.31). This is a bug -- please report it at https://github.com/rvben/rumdl/issues and include the panic message printed above.

Exit code unchanged (2).

No behavior change beyond the message text: catch_unwind, the exit code and control flow stay the same.

I didn't add an automated test for this. The message is a fixed string plus env!("CARGO_PKG_VERSION"), which fails to compile on its own if that were ever missing, and there's no existing way to trigger a real panic through the CLI to exercise this path (the historical panic reports, #85, #87, #154 and #168, were all UTF-8 char-boundary bugs already fixed in earlier releases). I verified the change by temporarily inserting a panic!() in the closure, confirming the output above, and reverting it before committing.

Local checks:

• cargo build
• cargo fmt --check
• cargo clippy --workspace --lib --bins --tests -- -D warnings -D clippy::uninlined_format_args