refactor: flatten repository structure to root and archive research notes

docs/research/compatibility/api-v1.md

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+# CHFS v1 compatibility surface
+
+Keep legacy endpoints stable during migration.
+
+- session: login/logout
+- files: directory listing
+- shared: direct file access
+- newdir: create directory
+- rename: rename file/directory
+- rmfiles: delete
+- search: global search
+- upload/upload2: upload
+- newtext/updatetext: text file editing
+- preview: preview
+- exist: existence check
+- downloaddir: ZIP directory download
+- webdav: WebDAV

docs/research/compatibility/config-v1.md

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+# Known legacy configuration
+
+path=/data
+image.preview=true
+file.remove=3
+
+Known additional keys from binary/config ecosystem:
+session.timeout
+folder.download
+folder.leaf.download
+rule.default
+rule.r
+rule.w
+rule.d
+html.title
+html.notice
+ssl.cert
+ssl.key