packaging/web: document what page host mode trusts, the limits on host files, and the stable event names

packaging/web/README.md

@@ -197,6 +197,12 @@ await init({ module_or_path: "./figcraft-web-<hash>_bg.wasm" });
   present it takes precedence, so a same-origin framed page uses the parent bridge, while an
   unframed or cross-origin page gets no bridge and keeps normal downloads.
 
+### Trust and limits
+
+- **Who is trusted.** In page host mode the page and FigCraft share one document, so any script on the page can drive the app: open files in it and receive what it saves. Use this mode only for a page you control. The iframe mode (`?host=parent`) answers only the parent window, on the page's own origin.
+- **Files from the host are untrusted input.** An opened file goes through the same reading as one picked with File ▸ Open or dropped on the window, with the same format limits; before it is copied into the app's memory, its size is checked against the host open limit (512 MB) and a larger file is refused, and a name is cut to 255 characters.
+- **The event and message names are the interface.** `figcraft:ready`, `figcraft:open`, `figcraft:save`, `figcraft:dirty` and `figcraft:failed`, and the fields they carry (the same as the `?host=parent` messages), stay stable; changing one is a breaking change.
+
 ## Renderer selection and fallback flags
 
 FigCraft renders with wgpu. It uses **WebGPU** when the browser has it and falls back to