Fix DbWriter transaction scope races

changelog.d/unreleased/1733.fixed.md

@@ -0,0 +1,16 @@
+---
+category: fixed
+issues:
+  - 1733
+affected:
+  - src/CodeIndex/Database/DbWriter.cs
+  - tests/CodeIndex.Tests/ConcurrencyTests.cs
+---
+
+## English
+
+- **DbWriter transaction depth is serialized for shared writers (#1733)** — transaction scopes on one writer now wait across threads while preserving same-thread nested savepoints, preventing depth races from leaking or mis-targeting savepoints.
+
+## 日本語
+
+- **共有 DbWriter の transaction depth を直列化しました (#1733)** — 1 つの writer 上の transaction scope はスレッド間で待機し、同一スレッドの nested savepoint は維持するため、depth race による savepoint 漏れや誤対象化を防ぎます。

changelog.d/unreleased/1741.fixed.md

@@ -0,0 +1,16 @@
+---
+category: fixed
+issues:
+  - 1741
+affected:
+  - src/CodeIndex/Database/DbWriter.cs
+  - tests/CodeIndex.Tests/DatabaseTests.cs
+---
+
+## English
+
+- **Savepoint cleanup failures are now diagnosable (#1741)** — savepoint scopes now throw an explicit invalid-operation error when their SQLite connection is missing and log cleanup rollback failures before best-effort disposal continues.
+
+## 日本語
+
+- **savepoint cleanup の失敗を診断できるようにしました (#1741)** — savepoint scope は SQLite connection が欠けている場合に明示的な invalid-operation error を投げ、cleanup rollback の失敗は best-effort dispose を続ける前にログへ残します。

changelog.d/unreleased/1783.fixed.md

@@ -0,0 +1,16 @@
+---
+category: fixed
+issues:
+  - 1783
+affected:
+  - src/CodeIndex/Database/DbWriter.cs
+  - tests/CodeIndex.Tests/DatabaseTests.cs
+---
+
+## English
+
+- **DbWriter transaction depth now survives begin failures (#1783)** — failed SQLite transaction or savepoint starts no longer leave `_transactionDepth` advanced without a matching active transaction.
+
+## 日本語
+
+- **DbWriter の transaction depth が begin 失敗後も整合するようになりました (#1783)** — SQLite transaction / savepoint の開始に失敗しても、対応する active transaction がないまま `_transactionDepth` だけが進む状態を残しません。

changelog.d/unreleased/2676.fixed.md

@@ -9,8 +9,8 @@ affected:
 
 ## English
 
-- **Ready-bit updates no longer start raw nested SQLite transactions (#2676)** — concurrent writers now use provider-managed immediate transactions for `PRAGMA user_version` updates.
+- **Ready-bit stamping no longer starts nested SQLite transactions on shared writers (#2676)** — `SetReadyBit` now shares the writer transaction gate and uses provider-managed immediate transactions, so concurrent calls wait for an active scope before updating `PRAGMA user_version`.
 
 ## 日本語
 
-- **ready-bit 更新で raw な nested SQLite transaction を開始しないようになりました (#2676)** — concurrent writer は `PRAGMA user_version` 更新に provider-managed immediate transaction を使います。
+- **ready-bit stamp が共有 writer 上で nested SQLite transaction を開始しないようにしました (#2676)** — `SetReadyBit` は writer transaction gate と provider-managed immediate transaction を使い、active scope が終わるまで待ってから `PRAGMA user_version` を更新します。