* feat(compiler): add compile start/complete notifications
Problem: No user-facing feedback when compilation starts or finishes.
Long-running compilers like pandoc with `--embed-resources` leave the
user staring at nothing for 15+ seconds.
Solution: Notify "compiling..." at compile start and "compilation
complete" on success. The initial `toggle` call uses a combined
"compiling with <name>..." message to avoid stacking two notifications.
* refactor(presets): use `--katex` instead of `--embed-resources --mathml`
Problem: `--embed-resources` with `--mathml` caused pandoc to inline all
assets at compile time, adding ~15s per save for KaTeX-heavy documents.
Solution: Default to `--katex`, which inserts a CDN `<script>` tag and
defers rendering to the browser. Users can opt into `--embed-resources`
or `--mathml` via `extra_args`.
* docs(presets): rewrite math rendering section for `--katex` default
* refactor(compiler): simplify notification flow, add failure notify
Problem: `compile()` used an `opts.silent` escape hatch so `toggle()`
could suppress duplicate notifications. Compilation failures had no
user-facing notification.
Solution: Remove `opts.silent` — `compile()` unconditionally notifies
on start, success, and failure. `toggle()` no longer emits its own
message.
* feat(compiler): add per-recompile notifications for long-running providers
Problem: long-running providers like `typst watch` had no per-recompile
feedback — the process stays alive and individual success/failure was
never reported.
Solution: add a persistent `output_watcher` fs_event that fires
"compilation complete" on every output mtime bump, and track
`has_errors` on `BufState` so stderr diagnostics trigger a one-shot
"compilation failed" notification. `diagnostic.set()` now returns the
diagnostic count to support this flow.
* ci: format
* chore: remove testing files
