fix(parse): position-independent inline metadata parsing (#164)

Problem: `parse.body()` scanned tokens right-to-left and broke on the
first non-metadata token, so metadata only worked at the trailing end
of a line. `due:tomorrow Fix the bug` silently failed to parse the
due date.

Solution: Replace the right-to-left `while` loop with a single
left-to-right pass that extracts metadata tokens from any position.
Duplicate metadata tokens are dropped with a `log.warn`. Update docs
and tests accordingly.

spec/parse_spec.lua

@@ -48,16 +48,35 @@ describe('parse', function()
       assert.are.equal('Errands', meta.category)
     end)
 
-    it('first occurrence wins for duplicate keys', function()
+    it('first occurrence wins for duplicate keys and warns', function()
+      local warnings = {}
+      local orig = vim.notify
+      vim.notify = function(m, level)
+        if level == vim.log.levels.WARN then
+          table.insert(warnings, m)
+        end
+      end
       local desc, meta = parse.body('Buy milk due:2026-03-15 due:2026-04-01')
+      vim.notify = orig
       assert.are.equal('Buy milk', desc)
       assert.are.equal('2026-03-15', meta.due)
+      assert.are.equal(1, #warnings)
+      assert.truthy(warnings[1]:find('duplicate', 1, true))
     end)
 
-    it('drops identical duplicate metadata tokens', function()
+    it('drops identical duplicate metadata tokens and warns', function()
+      local warnings = {}
+      local orig = vim.notify
+      vim.notify = function(m, level)
+        if level == vim.log.levels.WARN then
+          table.insert(warnings, m)
+        end
+      end
       local desc, meta = parse.body('Buy milk due:tomorrow due:tomorrow')
+      vim.notify = orig
       assert.are.equal('Buy milk', desc)
       assert.are.equal(os.date('%Y-%m-%d', os.time() + 86400), meta.due)
+      assert.are.equal(1, #warnings)
     end)
 
     it('stops at non-meta token', function()