io: files kept in memory in the browser build, recovery over them

The browser has no file system a page can call synchronously. On wasm32,
fieldcraft_io::fs now reads and writes files kept in memory (memory.rs,
after SolidCraft's vfs): the web host puts picked and dropped files in
and takes out what commands wrote. The recovery folder works over the
same calls (list, remove, is_file), with in-memory entry locks and a
host-provided clock on wasm, so autosave and File > Recover work in the
browser as on the desktop. Native behaviour is unchanged.

Part of #119

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

crates/io/src/lib.rs

@@ -18,11 +18,12 @@
 //! capped before anything is allocated, every part's checksum is verified, and the model is
 //! rebuilt through the same checked methods commands use, so a file cannot hold what commands
 //! would refuse. Reading and writing work on bytes (they build for the browser); only [`fs`],
-//! [`recovery`] and [`roots`] touch the file system.
+//! [`recovery`] and [`roots`] touch the file system, which in the browser is [`memory`].
 #![deny(clippy::unwrap_used, clippy::expect_used, clippy::panic, clippy::unimplemented, clippy::todo, clippy::unreachable)]
 #![forbid(unsafe_code)]
 
 pub mod fs;
+pub mod memory;
 pub mod mesh;
 pub mod recovery;
 pub mod roots;