fix: correct buffer parser and timestamp test

Problem: parse_buffer classified /id/-prefixed task lines as headers
because '/' matches the '^%S' header pattern. Store timestamp test
was flaky when add and update ran within the same second.

Solution: check for task line patterns (id prefix or 2-space indent)
before falling through to the header branch. Backdate the initial
modified timestamp in the store update test.

lua/todo/diff.lua

@@ -26,36 +26,34 @@ function M.parse_buffer(lines)
   local current_category = nil
 
   for i, line in ipairs(lines) do
+    local id, body = line:match('^/(%d+)/(  .+)$')
+    if not id then
+      body = line:match('^(  .+)$')
+    end
     if line == '' then
       table.insert(result, { type = 'blank', lnum = i })
+    elseif id or body then
+      local stripped = body:match('^  (.+)$') or body
+      local priority = 0
+      if stripped:match('^! ') then
+        priority = 1
+        stripped = stripped:sub(3)
+      end
+      local description, metadata = parse.body(stripped)
+      if description and description ~= '' then
+        table.insert(result, {
+          type = 'task',
+          id = id and tonumber(id) or nil,
+          description = description,
+          priority = priority,
+          category = metadata.cat or current_category or config.get().default_category,
+          due = metadata.due,
+          lnum = i,
+        })
+      end
     elseif line:match('^%S') then
       current_category = line
       table.insert(result, { type = 'header', category = line, lnum = i })
-    else
-      local id, body = line:match('^/(%d+)/(  .+)$')
-      if not id then
-        body = line:match('^(  .+)$')
-      end
-      if body then
-        local stripped = body:match('^  (.+)$') or body
-        local priority = 0
-        if stripped:match('^! ') then
-          priority = 1
-          stripped = stripped:sub(3)
-        end
-        local description, metadata = parse.body(stripped)
-        if description and description ~= '' then
-          table.insert(result, {
-            type = 'task',
-            id = id and tonumber(id) or nil,
-            description = description,
-            priority = priority,
-            category = metadata.cat or current_category or config.get().default_category,
-            due = metadata.due,
-            lnum = i,
-          })
-        end
-      end
     end
   end