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.

spec/compiler_spec.lua

@@ -55,6 +55,14 @@ describe('compiler', function()
         end,
       })
 
+      local notified = false
+      local orig = vim.notify
+      vim.notify = function(msg)
+        if msg:find('compiling') then
+          notified = true
+        end
+      end
+
       local provider = { cmd = { 'echo', 'ok' } }
       local ctx = {
         bufnr = bufnr,
@@ -64,7 +72,9 @@ describe('compiler', function()
       }
 
       compiler.compile(bufnr, 'echo', provider, ctx)
+      vim.notify = orig
       assert.is_true(fired)
+      assert.is_true(notified)
 
       vim.wait(2000, function()
         return process_done(bufnr)