feat(diff): propagate recurrence through buffer reconciliation

Problem: the diff layer does not extract or apply recurrence fields,
so rec: tokens written in the buffer are silently ignored on :w.

Solution: add rec and rec_mode to ParsedEntry, extract them in
parse_buffer(), and pass them through create and update paths in
apply().

lua/pending/diff.lua

@@ -10,6 +10,8 @@ local store = require('pending.store')
 ---@field status? string
 ---@field category? string
 ---@field due? string
+---@field rec? string
+---@field rec_mode? string
 ---@field lnum integer
 
 ---@class pending.diff
@@ -48,6 +50,8 @@ function M.parse_buffer(lines)
           status = status,
           category = metadata.cat or current_category or config.get().default_category,
           due = metadata.due,
+          rec = metadata.rec,
+          rec_mode = metadata.rec_mode,
           lnum = i,
         })
       end
@@ -90,6 +94,8 @@ function M.apply(lines)
           category = entry.category,
           priority = entry.priority,
           due = entry.due,
+          recur = entry.rec,
+          recur_mode = entry.rec_mode,
           order = order_counter,
         })
       else
@@ -112,6 +118,14 @@ function M.apply(lines)
           task.due = entry.due
           changed = true
         end
+        if task.recur ~= entry.rec then
+          task.recur = entry.rec
+          changed = true
+        end
+        if task.recur_mode ~= entry.rec_mode then
+          task.recur_mode = entry.rec_mode
+          changed = true
+        end
         if entry.status and task.status ~= entry.status then
           task.status = entry.status
           if entry.status == 'done' then
@@ -135,6 +149,8 @@ function M.apply(lines)
         category = entry.category,
         priority = entry.priority,
         due = entry.due,
+        recur = entry.rec,
+        recur_mode = entry.rec_mode,
         order = order_counter,
       })
     end