Add host-structured imports

doc/manual/souce/design/embedding-api.md

@@ -1,7 +1,7 @@
 # Embedding API
 
 Decodal core can be embedded without giving the core crate access to a filesystem.
-The host supplies imported sources through `SourceLoader` and may also provide global bindings through the host prelude API.
+The host supplies imports through `ImportLoader` and may also provide global bindings through the host prelude API.
 
 ## Host prelude
 
@@ -131,11 +131,32 @@ Supported field attributes are intentionally small:
 The derive does not add host callbacks or reflection.
 It only generates schema construction and typed decoding code.
 
-## SourceLoader and prelude together
+## ImportLoader and prelude together
 
-`SourceLoader` and host prelude bindings are independent mechanisms.
+`ImportLoader` and host prelude bindings are independent mechanisms.
 
-- Use `SourceLoader` when user sources should explicitly import host-provided modules.
+- Use `ImportLoader` when user sources should explicitly import host-provided sources or values.
 - Use prelude bindings when host-provided schemas or constants should be globally available.
 
 Both mechanisms share the same runtime evaluator, thunk model, and materialization rules.
+
+## Structured imports
+
+`ImportLoader::load` returns either `LoadedImport::Source` or `LoadedImport::Value`.
+The value variant carries a `HostValue`, allowing a host to parse non-Decodal resources such as Markdown into an application-specific structure.
+
+```text
+import "./post.md"
+  -> host parses content
+  -> LoadedImport::Value(
+       { frontmatter: {...}, body: "..." }
+     )
+  -> Engine internalizes HostValue
+  -> normal composition and materialization
+```
+
+The core does not select content types or bundle Markdown/frontmatter parsers.
+The loader owns path resolution, media or extension dispatch, parsing rules, and parse diagnostics.
+The stable loader key is also used to cache structured imports.
+
+`load` is the single import hook: loaders dispatch by extension, media type, or another host-defined rule and return the appropriate variant directly.