feat: add statusline function (#10)

Problem: no way to expose compiling/watching state to statusline
plugins like lualine or heirline without polling status() and
formatting it manually.

Solution: add `require('preview').statusline()` that returns
'compiling', 'watching', or '' for direct use in statusline components.

spec/init_spec.lua

@@ -100,4 +100,12 @@ describe('preview', function()
       helpers.delete_buffer(bufnr)
     end)
   end)
+
+  describe('statusline', function()
+    it('returns empty string when idle', function()
+      local bufnr = helpers.create_buffer({})
+      assert.are.equal('', preview.statusline(bufnr))
+      helpers.delete_buffer(bufnr)
+    end)
+  end)
 end)