Performance, threads and wasm

Do not block the window

• Work that can take long (a feature computation, a save, a file load, reading a picture) runs beside the window and is applied when done. Existing examples: crates/ui-egui/src/save_job.rs, crates/ui-egui/src/file_load.rs, and the agent path in docs/control-protocol.md ("Slow commands and the window").
• With the preference "Keep the window responsive while agents run commands" (agent_off_thread) on, a design change that takes longer than about 150 ms is evaluated ahead on a copy of the design, then run for real reusing that result. A person's edit made meanwhile goes first; Stop drops the computation and nothing is applied. Keep that contract when you add a long-running command.
• file.export and file.save_mesh change nothing in the design: they run on a copy and nothing waits behind them.
• Test it: crates/ui-egui/src/apply_off_thread_tests.rs drives a long feature through the harness; never depend on wall-clock timing in a test.

Measure before you claim a speed-up

• Profile first. Time with solidcraft-cli from main and from your branch, alternating runs, after the load is low. On a shared machine check cut -d' ' -f1 /proc/loadavg and wait while it is over 24; build with nice.
• Report medians with the range, the machine, and the cases. The pattern to follow is docs/perf.md: what was slow, why, the result, and proof that nothing changed (same bodies, volumes, areas and triangle counts).
• A speed-up never skips work on a mesh's box: decide with the exact bounds (Body::exact_bounds, grown by the record's tolerance as apart in crates/kernel/src/ops.rs does). A mesh is for display, export, previews and mesh bodies.
• Do not rewrite kernel code for speed: the kernel is being replaced, and kernel work is paused. Report a slow case with its file under the kernel-perf label (CONTRIBUTING.md, "Where help is wanted most").
• Pull requests run only the tests for what they touch; geometry-heavy tests run in the merge queue. Use cargo xtask test with the ci profile locally (skills/solidcraft-tests/SKILL.md).

The browser build

• Everything below the apps must build for wasm32-unknown-unknown: cargo xtask wasm checks every library crate and the web app, and is part of cargo xtask ci. Do not add a dependency that cannot.
• Application code is Rust only. The browser requires two pieces of JavaScript: the service worker apps/solidcraft-web/sw.js and the small inline scripts in apps/solidcraft-web/index.html. Keep them minimal; cargo xtask web-offline-test covers the service worker.
• Python 3.11+ is allowed only for asset authoring, lint and review tools (assets/AGENTS.md), never as an application dependency.
• Design notes for the browser runtime: docs/browser-api.md.