fix(p1-integration): validate.rs allows _schemas/fragments $ref + drop additionalProperties on fragment-composed atom schemas

Two P1↔E1-audit-wave integration regressions caught by kei-runtime
invoke_real_atom test.

1. LocalFileResolver (E1 SSRF hardening) rejected $ref to
   _schemas/fragments/ because the dir is OUTSIDE atom's schema parent.
   Fix: extend LocalFileResolver with `find_fragments_root()` — walks up
   from schema root looking for `_schemas/fragments/`. If found, allow
   $ref under EITHER schema root OR fragments root. Still rejects
   arbitrary filesystem $ref.

2. jsonschema injection of absolute $id now ALSO applied to fragment
   schemas loaded via LocalFileResolver.resolve(). Without this, a
   fragment declaring `$id: "_schemas/fragments/titled.json"` (relative)
   was resolved against parent schema's absolute $id, producing double
   prefix `_schemas/fragments/_schemas/fragments/titled.json`.

3. create-input.json + create-output.json had `additionalProperties:
   false` alongside `allOf: [$ref <fragment>]`. Draft-07 gotcha:
   additionalProperties at this level does NOT see properties inherited
   from $ref-ed fragment — caused 'title' unexpected rejection. Dropped
   the constraint on 2 fragment-composed schemas; kept on 4 standalone
   ones (search-input/output + add-dependency-input/output).

Tests: kei-runtime 5/5 green; integration test passes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

_primitives/_rust/kei-runtime/src/validate.rs

@@ -82,7 +82,9 @@ fn inject_absolute_id(schema: &mut Value, schema_path: &Path) {
 }
 
 /// `$ref` resolver that rejects every scheme except `file://`, AND rejects
-/// any path that is not inside `root` (canonicalised).
+/// any path that is not inside `root` OR the shared `_schemas/fragments/` dir
+/// (canonicalised). The fragments dir is resolved by walking up from `root`
+/// until a sibling `_schemas/fragments/` is found or we reach filesystem root.
 #[derive(Debug)]
 pub struct LocalFileResolver {
     root: PathBuf,
@@ -92,6 +94,20 @@ impl LocalFileResolver {
     pub fn new(root: PathBuf) -> Self {
         Self { root }
     }
+
+    /// Walk up from root to find workspace's `_schemas/fragments/`. Returns
+    /// canonicalised path if found. Allows atom schemas to $ref shared
+    /// fragments without opening the entire filesystem.
+    fn find_fragments_root(&self) -> Option<PathBuf> {
+        let mut cur = self.root.as_path();
+        loop {
+            let candidate = cur.join("_schemas").join("fragments");
+            if let Ok(canon) = candidate.canonicalize() {
+                return Some(canon);
+            }
+            cur = cur.parent()?;
+        }
+    }
 }
 
 impl SchemaResolver for LocalFileResolver {
@@ -117,17 +133,33 @@ impl SchemaResolver for LocalFileResolver {
             .root
             .canonicalize()
             .map_err(|e| anyhow::anyhow!("canonicalize root {}: {e}", self.root.display()))?;
-        if !canon.starts_with(&root_canon) {
+        let fragments_canon = self.find_fragments_root();
+        let in_root = canon.starts_with(&root_canon);
+        let in_fragments = fragments_canon
+            .as_ref()
+            .map(|f| canon.starts_with(f))
+            .unwrap_or(false);
+        if !in_root && !in_fragments {
             return Err(anyhow::anyhow!(
-                "file $ref escapes schema root: {} not under {}",
+                "file $ref escapes both schema root and fragments dir: {} not under {} or _schemas/fragments/",
                 canon.display(),
                 root_canon.display()
             ));
         }
         let f = std::fs::File::open(&canon)
             .map_err(|e| anyhow::anyhow!("open {}: {e}", canon.display()))?;
-        let doc: Value = serde_json::from_reader(f)
+        let mut doc: Value = serde_json::from_reader(f)
             .map_err(|e| anyhow::anyhow!("parse {}: {e}", canon.display()))?;
+        // Override any relative `$id` in the loaded fragment with its
+        // absolute file:// URL. Without this, a fragment declaring e.g.
+        // `$id: "_schemas/fragments/titled-content.json"` would be
+        // resolved relative to the parent schema's $id by jsonschema,
+        // producing a doubled prefix (`_schemas/fragments/_schemas/...`).
+        if let Some(obj) = doc.as_object_mut() {
+            if let Ok(abs_url) = Url::from_file_path(&canon) {
+                obj.insert("$id".to_string(), Value::String(abs_url.to_string()));
+            }
+        }
         Ok(Arc::new(doc))
     }
 }

_primitives/_rust/kei-task/atoms/schemas/create-input.json

@@ -15,7 +15,6 @@
     },
     "milestone_id": { "type": "integer", "minimum": 1 }
   },
-  "additionalProperties": false,
   "examples": [
     { "title": "Fix auth bug", "priority": "high" },
     { "title": "Refactor router", "description": "Split monolith", "priority": "medium" }

_primitives/_rust/kei-task/atoms/schemas/create-output.json

@@ -6,7 +6,6 @@
   "allOf": [
     { "$ref": "../../../../../_schemas/fragments/entity-base.json" }
   ],
-  "additionalProperties": false,
   "examples": [
     { "id": 42, "created_at": 1714000000 }
   ]