fix(persist): warn on session data decode failure instead of silent fallback

Add path-prefixed warning when JSON decode fails in store.read and
store.read_sync, so users can identify corrupted session files
instead of seeing sessions silently disappear.

Author: mhiro2

lua/peekstack/persist/store.lua

@@ -72,6 +72,7 @@ function M.read(scope, opts)
         vim.schedule(function()
           local ok, decoded = pcall(vim.json.decode, data)
           if not ok or type(decoded) ~= "table" then
+            notify.warn("Failed to decode session data: " .. path)
             on_done(empty_data())
             return
           end
@@ -104,6 +105,7 @@ function M.read_sync(scope)
 
   local ok, decoded = pcall(vim.json.decode, data)
   if not ok or type(decoded) ~= "table" then
+    notify.warn("Failed to decode session data: " .. path)
     return empty_data()
   end
   return decoded

tests/persist_store_spec.lua

@@ -120,12 +120,24 @@ describe("peekstack.persist.store", function()
     assert.same(data, result)
   end)
 
-  it("returns empty data for invalid JSON", function()
+  it("returns empty data and warns for invalid JSON", function()
     local path = fs.scope_path(test_scope)
     write_raw_and_wait(path, "{ invalid json")
 
+    local warnings = {}
+    local original_notify = vim.notify
+    vim.notify = function(msg, level)
+      if level == vim.log.levels.WARN then
+        table.insert(warnings, msg)
+      end
+    end
+
     local result = read_and_wait(test_scope)
+
+    vim.notify = original_notify
     assert.same({ version = 2, sessions = {} }, result)
+    assert.is_true(#warnings > 0, "should warn about decode failure")
+    assert.is_true(warnings[1]:find("Failed to decode", 1, true) ~= nil)
   end)
 
   it("read_sync returns data for valid store content", function()
@@ -144,12 +156,24 @@ describe("peekstack.persist.store", function()
     assert.same(data, result)
   end)
 
-  it("read_sync returns empty data for invalid JSON", function()
+  it("read_sync returns empty data and warns for invalid JSON", function()
     local path = fs.scope_path(test_scope)
     write_raw_and_wait(path, "{ invalid json")
 
+    local warnings = {}
+    local original_notify = vim.notify
+    vim.notify = function(msg, level)
+      if level == vim.log.levels.WARN then
+        table.insert(warnings, msg)
+      end
+    end
+
     local result = store.read_sync(test_scope)
+
+    vim.notify = original_notify
     assert.same({ version = 2, sessions = {} }, result)
+    assert.is_true(#warnings > 0, "should warn about decode failure")
+    assert.is_true(warnings[1]:find("Failed to decode", 1, true) ~= nil)
   end)
 
   it("write_sync stores data that can be read back", function()

tests/setup_idempotent_spec.lua

@@ -35,7 +35,6 @@ describe("peekstack.setup idempotent", function()
       providers = {
         marks = {
           enable = true,
-          scope = "buffer",
         },
       },
     })