fix(fermata): .botignore as fallback root, not ignored; extract SVGs to files

- Walk-up: .botignore no longer stops the search but is remembered as a
  fallback when no strong marker (.git, botignore.toml, .botignore.toml)
  is found — prevents fail-open regression for projects without .git
- Extract inline SVGs to policy-layers.svg and architecture.svg
- READMEs reference SVGs via <img> tags

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/core_project.rs

@@ -51,18 +51,36 @@ fn botignore_alone_does_not_stop_walk() {
 }
 
 #[test]
-fn botignore_only_returns_none() {
-    // If only .botignore exists with no real root marker, no project root is found.
+fn botignore_used_as_fallback() {
+    // If only .botignore exists (no strong marker), it serves as a fallback
+    // root so that policy is still enforced.
     let tmp = TempDir::new().unwrap();
     let root = tmp.path();
     fs::create_dir_all(root.join("sub")).unwrap();
-    fs::write(root.join(".botignore"), "").unwrap();
+    fs::write(root.join(".botignore"), "*.secret").unwrap();
 
     let target = root.join("sub/file.rs");
     fs::write(&target, "").unwrap();
 
-    // .botignore alone should NOT define a project root
-    assert!(find_project_root(&target).is_none());
+    let found = find_project_root(&target).unwrap();
+    assert_eq!(found, root);
+}
+
+#[test]
+fn strong_marker_preferred_over_botignore_fallback() {
+    // .botignore at a/b/, .git at root — walk past .botignore, use root.
+    let tmp = TempDir::new().unwrap();
+    let root = tmp.path();
+    fs::create_dir_all(root.join("a/b/c")).unwrap();
+    fs::create_dir_all(root.join(".git")).unwrap();
+    fs::write(root.join("a/b/.botignore"), "*.key").unwrap();
+
+    let target = root.join("a/b/c/file.rs");
+    fs::write(&target, "").unwrap();
+
+    // Should find root (with .git), not a/b (with .botignore)
+    let found = find_project_root(&target).unwrap();
+    assert_eq!(found, root);
 }
 
 #[test]