lsp: document every built-in member, complexity included

[simontreanor]

Follow-up to #42, and groundwork for the four remaining sweep PRs.

Big-O was written down in DESIGN prose and in comments beside the schemes, which is nowhere a user can see it. Hover on a built-in showed its type and effect and no prose at all, since doc comments only ever came from a user's own declarations. Completion items carried a label and nothing else — not even the signature.

types::MEMBER_DOCS now gives all 151 built-in members a one-line description, keyed by the name a user writes. The server appends it to hover below the type, exactly where a ## doc already goes, and attaches it to completion items along with the rendered signature as detail.

List.contains        a -> List a -> bool
                     Whether the list holds this element. O(n) linear scan — Set.contains is O(1).

List.updateAt        int -> 'a -> List 'a -> List 'a
                     A fresh list with one index replaced. O(n) — it copies; an index outside
                     the list changes nothing.

Complexity is stated wherever it is not obvious or not what a reader would assume, which is the whole point: a List backed by a Python list makes cost easy to misjudge. contains is a linear scan where Set.contains is O(1); updateAt/insertAt/removeAt copy; distinct hashes; Map.add copies, though a fold that builds a map lowers to an in-place loop.

A user's own declaration wins over a built-in of the same name — there is a test where a user function called max does not inherit the prelude's documentation.

Signatures cannot drift: they are rendered once on first use by seeding a fresh environment and showing each scheme, rather than being written out by hand a second time. Constraints are not spelled in the signature; where comparison or num matters, the member's doc line says so.

Three tests pin the table to the prelude constants — every member is documented, every entry names a real member, every member has a signature. That is the part that pays off later: the four remaining sweep PRs (Seq, Map/Set, String, Option/Result, ~60 members) cannot add an undocumented member, so the enforcement is in place before the members arrive rather than leaving them to backfill.

Plus four LSP tests: hover on a qualified member, hover on a global, a user declaration shadowing a built-in name, and a completion item carrying both signature and documentation.