feat(compiler): debounce compilation in toggle mode

Problem: in toggle mode, each BufWritePost immediately spawned a new
compilation, killing any in-flight process. Rapid saves wasted cycles
on compilers like latexmk.

Solution: add a 500ms debounce timer per buffer. The BufWritePost
callback starts/restarts the timer instead of compiling immediately.
Timers are cleaned up on unwatch and BufWipeout.

lua/preview/compiler.lua

@@ -12,6 +12,11 @@ local watching = {}
 ---@type table<integer, true>
 local opened = {}
 
+---@type table<integer, uv_timer_t>
+local debounce_timers = {}
+
+local DEBOUNCE_MS = 500
+
 ---@param val string[]|fun(ctx: preview.Context): string[]
 ---@param ctx preview.Context
 ---@return string[]
@@ -171,8 +176,15 @@ function M.toggle(bufnr, name, provider, ctx_builder)
   local au_id = vim.api.nvim_create_autocmd('BufWritePost', {
     buffer = bufnr,
     callback = function()
+      if debounce_timers[bufnr] then
+        debounce_timers[bufnr]:stop()
+      else
+        debounce_timers[bufnr] = vim.uv.new_timer()
+      end
+      debounce_timers[bufnr]:start(DEBOUNCE_MS, 0, vim.schedule_wrap(function()
         local ctx = ctx_builder(bufnr)
         M.compile(bufnr, name, provider, ctx)
+      end))
     end,
   })
 
@@ -198,6 +210,11 @@ function M.unwatch(bufnr)
     return
   end
   vim.api.nvim_del_autocmd(au_id)
+  if debounce_timers[bufnr] then
+    debounce_timers[bufnr]:stop()
+    debounce_timers[bufnr]:close()
+    debounce_timers[bufnr] = nil
+  end
   watching[bufnr] = nil
   log.dbg('unwatched buffer %d', bufnr)
 end
@@ -254,6 +271,7 @@ M._test = {
   active = active,
   watching = watching,
   opened = opened,
+  debounce_timers = debounce_timers,
 }
 
 return M