Resolve JSON-LD @id references, accept schema.org type IRIs, refuse empty durations

Review findings on the Recipe extractor:

- An image or publisher given as an {"@id": ...} reference now resolves to
  the node with that @id at a script's top level, in a top-level array or
  in an @graph, in any of the page's scripts. An image reference no node
  has, or one naming two nodes, is ErrMalformedRecipe; an unresolved
  publisher falls through to og:site_name and the host as before.
- @type matches the term or the absolute https:// or http://schema.org IRI,
  as a string or in an array, for Recipe, HowToStep and HowToSection.
- A present but empty or blank prepTime, cookTime or totalTime is an invalid
  ISO-8601 duration (ErrMalformedRecipe); an absent or null one is still
  nil.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/ARCHITECTURE.md

@@ -227,16 +227,18 @@ automatic** (D-021).
   (`recipes.Extract`): a small targeted scan of the page's
   `application/ld+json` scripts and `og:site_name` (no HTML parser
   dependency), finding the Recipe node at a script's top level, in a
-  top-level array or in an `@graph`, with `@type` a string or an array. The
-  largest image (by ImageObject width × height; the first of equals) is
+  top-level array or in an `@graph`, with `@type` a string or an array of
+  terms or absolute schema.org IRIs (`https://schema.org/Recipe`). An image
+  or publisher given as an `{"@id": ...}` reference resolves to the node
+  with that `@id` anywhere in the page's JSON-LD. The largest image (by ImageObject width × height; the first of equals) is
   resolved to an absolute URL; instructions — one string of lines, or a
   list of strings, HowToSteps and HowToSections — are flattened in order;
   ISO-8601 prep/cook/total times become minutes, total derived as prep +
   cook when only those are given; the site name is the publisher's, else
   `og:site_name`, else the host. A page with no Recipe is `ErrNoRecipe`,
   with two is `ErrManyRecipes`; a shape schema.org does not allow — such as
-  a non-ISO-8601 duration like "20 minutes" — is `ErrMalformedRecipe`,
-  never guessed at.
+  a non-ISO-8601 duration like "20 minutes" or an empty one, or an image
+  `@id` no node has — is `ErrMalformedRecipe`, never guessed at.
 - Photos are cached locally (`photos`) and processed with
   `golang.org/x/image`.