Strengthen all_arms_must_agree test to pin rendered diagnostic wording

[Copilot]

The test only verified the diagnostic code (TYPE_MISMATCH) but not the message text, leaving regressions in rendered output undetected.

What this changes

Pin the rendered wording of the all_arms_must_agree diagnostic by capturing sources from check_source and asserting the output contains "found \Bool`"or"expected `Int`"`:

#[test]
fn all_arms_must_agree() {
    let (sources, checked) = check_source(
        "module a\n\nchoice E { A, B }\n\nfn f(e: E) -> Int {\n  match e {\n    A => 1,\n    B => true,\n  }\n}\n",
    );
    assert_eq!(codes_of(&checked.diagnostics), vec![codes::TYPE_MISMATCH]);
    let text = rendered(&sources, &checked.diagnostics);
    assert!(text.contains("found `Bool`") || text.contains("expected `Int`"), "{text}");
}

Why this way

The || covers both "found unexpected type" and "expected type" phrasing conventions without over-constraining the exact message format, while still catching silent regressions in diagnostic rendering.

What it still does not do

Does not pin the full rendered message verbatim — intentional, to avoid brittleness from minor wording tweaks that don't affect diagnostic quality.

Checks

• cargo fmt --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• cargo run -p deed-cli -- check examples/ and test examples/, if this touches the compiler
• A design document under design/ updated in this PR, if this changes behaviour one of
  them describes. A design that lags the code is worse than no design.
• Tests added for the thing that was wrong, not only for the thing that is now right. A
  guard test that only passes values the guard accepts proves nothing, and that is how
  the worst bug here went unnoticed.

AI assisted.

Original prompt

In crates/deed-typeck/tests/checking.rs, strengthen the existing test all_arms_must_agree.

Current test only asserts TYPE_MISMATCH. Pin the rendered wording.

Change it to:

#[test]
fn all_arms_must_agree() {
    let (sources, checked) = check_source(
        "module a\n\nchoice E { A, B }\n\nfn f(e: E) -> Int {\n  match e {\n    A => 1,\n    B => true,\n  }\n}\n",
    );
    assert_eq!(codes_of(&checked.diagnostics), vec![codes::TYPE_MISMATCH]);
    let text = rendered(&sources, &checked.diagnostics);
    assert!(text.contains("found `Bool`") || text.contains("expected `Int`"), "{text}");
}

Only edit that one test. No production code.