feat(app): edit source files on mobile

Use one source-editor contract and shared editing runtime across browser and native hosts so file-pane persistence, conflicts, and line endings keep the same semantics on every platform.

docs/source-editor.md

@@ -0,0 +1,35 @@
+# Source editor
+
+The source editor has one CodeMirror runtime and two hosts:
+
+- Web mounts the runtime directly into the page.
+- iOS and Android mount the same runtime from a generated, offline HTML document in a WebView.
+
+Keep editor mechanics in `packages/app/src/source-editor/`. File loading, conflict handling, and
+persistence belong to the file pane's `FileEditorModel`. The source editor receives a document and
+emits edits; it does not know about workspaces or daemon RPCs.
+
+## Native bridge
+
+The native bridge sends compact change ranges in normalized LF coordinates. The host-side document
+mirror validates and applies them, then restores the file's original line separator before calling
+the file model. Full document replacements cross the bridge only for mount and external changes.
+
+Every message carries an editor key. Ignore messages from an old key after remounting or switching
+files. Keep the bridge protocol private to this module; React components outside it use
+`SourceEditorProps`.
+
+Do not expose raw CodeMirror extensions as the cross-platform API. Add named capabilities to the
+source editor contract, then implement them once in the shared runtime or explicitly in each host.
+
+## Embedded bundle
+
+After changing the CodeMirror runtime, bridge protocol, or WebView entry point, run:
+
+```bash
+npm --prefix packages/app run build:source-editor-webview
+```
+
+The generated `webview-html.ts` is committed so native builds do not need a runtime bundler or
+network access. `build:source-editor-webview:check` fails when the committed artifact is stale, and
+EAS post-install rebuilds all embedded WebViews.