The IDL `types` array was emitted in HashMap iteration order by
spel-framework-core, which is non-deterministic across processes. Two
independent regenerations of the same source could therefore disagree on
type ordering, producing different bytes.
This is what makes the check-idl CI job flaky: a PR's committed IDL is
generated locally with one ordering, but CI regenerates with a different
ordering and the diff fails — including PRs that were green when posted
then breaking main after merge.
Sort the top-level `types` array by name before serializing so output is
byte-stable regardless of where idl-gen runs. Enable serde_json's
`preserve_order` feature so the Value round-trip preserves struct-field
key order (otherwise all keys would alphabetize and churn every artifact).
Only top-level `types` was unstable; variants and fields already follow
source order. Committed artifacts are unchanged — they happened to already
be in sorted order.
Move IDL files to artifacts/ and add a convention-based CI check that
discovers all programs via */methods/guest/src/bin/*.rs and fails if
any program is missing its IDL or has one that is out of date.
