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 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

docs/architecture.md

@@ -30,7 +30,7 @@ crates in strictly lower layers.
 | L2 | `fieldcraft-lbm` | Lattice Boltzmann flow solver on the CPU: D3Q19 two-relaxation-time scheme, bounce-back and slip walls, Zou–He inlets and outlets, obstacles voxelised from the face-tagged surface with exact predicates, momentum-exchange forces per face. Uses `mesh` for the predicates and the boundary surface. | started |
 | L3 | `fieldcraft-fsi` | Fluid–structure coupling between the L2 solvers (later). | planned |
 | L4 | `fieldcraft-doc` | The document: geometry, mesh, model, results, history. | planned |
-| L5 | `fieldcraft-io` | `.fieldcraft` files (docs/formats/fieldcraft.md): text model definition, binary mesh, geometry references with content hashes, the journal; atomic saves; autosave storage for crash recovery; read and write roots that confine an automation client's file access (`roots`). Bytes in, bytes out; the file system only in `fs`, `recovery` and `roots`. Result files later. | started |
+| L5 | `fieldcraft-io` | `.fieldcraft` files (docs/formats/fieldcraft.md): text model definition, binary mesh, geometry references with content hashes, the journal; atomic saves; autosave storage for crash recovery; read and write roots that confine an automation client's file access (`roots`). Bytes in, bytes out; the file system only in `fs`, `recovery` and `roots` (in the browser, files kept in memory: `memory`). Result files later. | started |
 | L6 | `fieldcraft-engine` | Commands (`CommandSpec`), `Session::execute` with rollback and the panic guard, scripts; every file a command names checked against the session's roots (`Session::file_access`); headless pictures of the session (`view`: model, geometry, deformed shape and contours on the CPU rasterizer). | started |
 | L7 | `fieldcraft-ui-egui`, `fieldcraft-mcp` | Desktop UI in the Craft family's layout (title bar with quick actions, workspace ribbon from the catalogue, browser, viewport with navigation bar and orientation gizmo, context panel, command search and command line, journal timeline; native file pickers as host services, background opening and solving; results display with legend and probe; offscreen capture of the whole window) and control channel requests (docs/control-protocol.md); MCP server over stdio (docs/mcp.md): tools over engine commands, a headless session or the running app through its control channel, read and write roots. | ui, mcp started |
 | testkit | `fieldcraft-testkit` | Test tooling, a dev-dependency only: the meshing test corpus (`tests/corpus/*/recipe.json`, analytic shapes) and its runner (`cargo xtask corpus`). | started |