Any Display error works, including Box<dyn Error> — and it already did

[zmaril]

Stacked on #7. Item 5, with a correction: the premise was wrong, and the
wrongness was mine.

I reported that a function which can fail two ways had to flatten to
Result<_, String> and litter itself with .map_err(|e| e.to_string()) — six
such calls in jawohl's surface. But jedem never inspected the error type.
Every backend renders failure as that language's own mechanism carrying the
error's Display text, so Box<dyn Error> and anyhow::Error have always
worked. jawohl's map_err calls were self-inflicted.

So this adds no feature. It proves the behaviour, documents it where someone
would look, and demonstrates it:

pub fn halve_parsed(text: &str) -> Result<i64, Box<dyn Error>> {
    let n: i64 = text.parse()?;              // one failure type
    if n % 2 != 0 { return Err(format!("{n} is odd").into()); }   // another
    Ok(n / 2)
}

hello.halve_parsed("10")      # 5
hello.halve_parsed("9")       # ValueError: 9 is odd
hello.halve_parsed("banana")  # ValueError: invalid digit found in string

jawohl's surface can now drop its six map_err calls — a change in the
jawohl repo, which I'll make there.

The fourth occurrence of one bug class

While adding the demo, cargo fmt broke the drift guard again — this time
collapsing a double blank line between two interfaces. The previous three were a
trailing space on an empty doc line, a trailing blank line at EOF, and the same
again per backend.

Patching the emitter a fourth time would have been the wrong response. The
invariants are now central: a normalise() step every backend's output
passes through, enforcing no trailing whitespace, no run of blank lines, exactly
one terminal newline. Tests assert each across Target::ALL, plus one that
generation is normalisation-stable — what jedem writes is what rustfmt would
leave alone.

39 tests, clippy clean, both round-trips passing.

Next and last of the series: #[derive(Enum)].

🤖 Generated with Claude Code

https://claude.ai/code/session_01HsDxLrGdx6nPaXkVEWkNvS