feat: compile notifications and long-running provider feedback (#55)

* 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

lua/preview/presets.lua

@@ -224,7 +224,7 @@ M.markdown = {
   ft = 'markdown',
   cmd = { 'pandoc' },
   args = function(ctx)
-    return { ctx.file, '-s', '--embed-resources', '--mathml', '-o', ctx.output }
+    return { ctx.file, '-s', '--katex', '-o', ctx.output }
   end,
   output = function(ctx)
     return (ctx.file:gsub('%.md$', '.html'))
@@ -249,8 +249,7 @@ M.github = {
       'gfm',
       ctx.file,
       '-s',
-      '--embed-resources',
-      '--mathml',
+      '--katex',
       '--css',
       'https://cdn.jsdelivr.net/gh/pixelbrackets/gfm-stylesheet@master/dist/gfm.css',
       '-o',