M3.14: project files name the version that wrote them, never save unreadable, keep their format

- `Project::to_file_json` writes `savedBy` (the EffectCraft version) ahead of the project and
  checks that the text reads back before anything is written: JSON has no NaN or infinity, so
  such a value made a file that saved but could never be opened. Save now fails with a message
  and the file on disk is left as it was.
- Opening a project saved by a newer EffectCraft warns that settings this version doesn't know
  are lost on saving; `file.open` reports `savedBy`. A file that isn't a project names itself in
  the error.
- Increment and Save keeps the extension (`.ecprojx` stays XML); Save a Copy writes XML for
  `.ecprojx` / `.xml` paths, like Save As.
- Docs: architecture (document model), parity.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/architecture.md

@@ -73,6 +73,14 @@ Everything animatable is a `Property` in the layer's `PropGroup` tree, addressed
 `transform/position`, `effects/#1/blurriness`, `masks/#1/feather`, or by `@uid`. Effects, masks,
 text animators and shape contents are groups in the same tree.
 
+A project file (`.ecproj`) is the project as readable JSON, headed by `savedBy` (the EffectCraft
+version that wrote it) and `schema`; `.ecprojx` is the same data as XML (File ▸ Save a Copy As
+XML). Saving goes through `Project::to_file_json`, which checks that the text reads back before
+anything is written (JSON can't hold NaN or infinity), and files are written atomically. Opening
+a file that a newer EffectCraft saved shows a warning: fields this version doesn't know are
+dropped if it saves the project. Save, Save As, Save a Copy and Increment and Save keep the
+file's format (`.ecprojx` stays XML).
+
 ## 4. Rendering
 
 `Renderer::comp_frame(comp, t)` walks the layers bottom to top. For each visible layer it renders

docs/parity.md

@@ -383,6 +383,15 @@ Browser in the web app (landed in M13.10: browser storage and File System Access
   wrong signatures and versions, fuel exhaustion, traps and bit-identical output; script
   `Socket` validates ports, and ScriptUI resource strings accept trailing array commas.
 
+## Update: M3.14 project files
+
+Project files name the version that wrote them (`savedBy`); opening one that a newer EffectCraft
+saved warns that what this version doesn't know is lost on saving (`file.open` reports
+`savedBy`). Saving checks that the file reads back first, so a value JSON can't hold (NaN)
+fails the save with a message instead of writing a project that can never be opened (the file
+on disk is left as it was). Increment and Save and Save a Copy keep an XML project (`.ecprojx`)
+XML; a file that isn't a project says so by name.
+
 ## Update: M3.13 menu bar
 
 Group Shapes (Ctrl+G) and Ungroup Shapes (Ctrl+Shift+G) moved to the Layer menu itself, where