feat: add ThreadListModel with batch append

QAbstractTableModel over query results, appended in batches so a large
query paints its first screenful immediately. Tag changes apply locally
for optimistic UI; reverting a failed write means calling applyTagChange
again with added and removed swapped, which the round-trip test pins.

Two additions to the drafted version:

- A ThreadIdRole, so a view's QModelIndex maps back to the thread id the
  worker speaks without every caller reaching around the model.
- data() checks its own row and column bounds. Qt will not hand out an
  out-of-range index and invalidates persistent ones on reset, so this is
  unreachable defence rather than a live path; the test says so instead of
  pretending to cover it.

Verified by mutation that the empty-batch guard, the ThreadIdRole, and the
full-row dataChanged range each fail exactly one test when removed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 5198429..5f01128 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -6,6 +6,7 @@ add_library(qtmaildir_lib STATIC
     htmlbuilder.cpp
     cidschemehandler.cpp
     notmuchworker.cpp
+    threadlistmodel.cpp
 )
 
 target_include_directories(qtmaildir_lib