* Add the format registry: one table of formats, extensions and options in crates/io.
The hosts' Open-dialog filters and the web accept list derive from it, and
registry tests pin it to the path predicates and the command docs.
* Fix format-registry tests that compared FormatSpec without Eq.
assert_eq on Option<&FormatSpec> does not compile; the none cases
only need is_none, matching the other rows that compare ids.