Report structured import origins in diagnostics

doc/manual/souce/design/diagnostics-and-fallback.md

@@ -11,6 +11,7 @@ Diagnostic {
   span: Span
   message: String
   labels: Vec<DiagnosticLabel>
+  notes: Vec<String>
 }
 
 DiagnosticLabel {
@@ -22,6 +23,7 @@ DiagnosticLabel {
 `span` は primary location を示す。
 表示時には `Span.source` を source id のまま出すのではなく、可能な限り file path や virtual file name に解決する。
 `labels` は同じ error に関係する追加 location を示す。
+`notes` は source location を持たない semantic context を示す。
 合成や materialize の失敗では、衝突した constraint、value、default、または処理中 field path を label に含める。
 
 代表的な diagnostic kind:
@@ -37,6 +39,9 @@ DiagnosticLabel {
 - match failure
 - materialization failure
 
+Structured imports use semantic provenance rather than synthetic source spans.
+Constraint failures report the imported value's stable key and logical field or array path alongside the source span of the Decodal constraint that rejected it.
+
 ## エラーは値ではない
 
 評価失敗は `RuntimeValue` ではなく `Diagnostic` を返す。

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

@@ -159,4 +159,7 @@ 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.
 
+When a structured value fails a Decodal constraint, the diagnostic keeps the Decodal constraint span and identifies the host value by its stable import key and logical value path, such as `content/post.md` and `frontmatter.draft`.
+`HostValue` does not need source spans: syntax diagnostics for the external format remain the loader's responsibility, while cross-value validation reports semantic provenance.
+
 `load` is the single import hook: loaders dispatch by extension, media type, or another host-defined rule and return the appropriate variant directly.