[P0] Option/Result aren't constructable at runtime without a local type declaration

[ecto]

The Errors and Types guides present Some/None/Ok/Err as built-in
constructors:

[match [Some 42]
  [Some x] x
  None     0]

In loon run (0.6), those forms fail with value is not callable
unless the user first declares the ADT at top level:

[type Option [Some a] None]

A missing map key also returns () (Unit), not None, so the
documented Option idiom doesn't even arise from get.

What to decide

Pick one canonical story and align everything to it:

• Option A: ship Option and Result in the prelude so the docs
  work as written, and make [get m :k] return None for a miss.
• Option B: keep the runtime as-is and rewrite every guide to show
  the [type Option …] declaration and ()-as-absence, plus a
  get-with-default pattern.

Acceptance criteria

• A runtime test that asserts the canonical story works as documented.
• Every code sample in web/src/pages/guide/errors.loon and
  web/src/pages/guide/types.loon runs under loon run verbatim.

Found while completing the hyperpolyglot ml column (clarkgrubb/hyperpolyglot#139).