Support durable JSONL persistence on Windows

docs/rfc/INDEX.md

@@ -150,6 +150,7 @@ Generated by `pnpm run gen-rfc-index` from the RFC tree — never edit by hand;
 | [Prompt variables and tool-guidance ownership](implemented/architecture/2026-07-05-prompt-variables-and-tool-guidance-ownership.md) | 2026-07-05 |
 | [Every LLM request is reconstructable from the session log](implemented/architecture/2026-07-05-reconstructable-requests.md) | 2026-07-05 |
 | [Subagent provider-lifecycle events — `subagent/provider-added` / `subagent/provider-removed`](implemented/architecture/2026-07-05-subagent-provider-lifecycle-events.md) | 2026-07-05 |
+| [Windows-native durable JSONL publication](implemented/architecture/2026-07-05-windows-jsonl-durable-publish.md) | 2026-07-05 |
 | [A shared timeout/deadline primitive, with hard-kill left to each capability](implemented/architecture/2026-07-06-timeout-deadline-library.md) | 2026-07-06 |
 | [Tool result retention library](implemented/architecture/2026-07-06-tool-result-retention-library.md) | 2026-07-06 |
 | [Tool-call timeout policy as a plugin](implemented/architecture/2026-07-07-tool-call-timeout-policy.md) | 2026-07-07 |

docs/rfc/implemented/architecture/2026-07-05-windows-jsonl-durable-publish.md

@@ -0,0 +1,33 @@
+# RFC: Windows-native durable JSONL publication
+
+Status: implemented
+
+## Problem
+
+`dsh-session-persistence-jsonl` publishes a session log lazily on the first append. The POSIX protocol writes a temp file, fsyncs it, links it to the final name, fsyncs the parent directory, and then removes the temp link. The parent-directory fsync is part of the durability contract: a crash after the namespace change must not lose the committed final name while leaving callers believing the session log materialized.
+
+Windows has atomic namespace operations, but Node does not expose a POSIX-equivalent parent-directory fsync contract there. Treating Windows directory sync failures as success would silently weaken a durable backend. The Windows path therefore needs a different publication primitive rather than a conditional inside the POSIX `syncDir` helper.
+
+## Decision
+
+The JSONL backend forks inside `materialize()` before any namespace mutation. Shared code computes the session directory, final log path, and initial JSONL bytes; POSIX and Windows then run separate publication protocols.
+
+POSIX keeps the existing protocol: create the root and cwd bucket with parent directory fsyncs, write and fsync a temp file, publish with `link()` so an existing final log is never overwritten, fsync the bucket directory, then remove the redundant temp hard link.
+
+Windows creates missing directories through a durable staging publish: create a random sibling directory, then publish it to the final directory name with `MoveFileExW(..., MOVEFILE_WRITE_THROUGH)` without `MOVEFILE_REPLACE_EXISTING` or `MOVEFILE_COPY_ALLOWED`. File materialization writes and fsyncs the temp log, then publishes that temp file to the final path with the same write-through `MoveFileExW` call and no replacement. `koffi` is the minimal Win32 bridge for this API surface; its install script is allowed in `pnpm-workspace.yaml` because the package ships the native loader and prebuilt platform modules.
+
+## Alternatives considered
+
+**Ignore Windows directory-sync failures.** Rejected because it reports a first append as durable without forcing the published namespace entry to stable storage.
+
+**Use `CreateHardLinkW`.** Rejected because hard links are filesystem-dependent, do not publish directories, and expose no write-through option.
+
+**Use replacement or transactional APIs.** `ReplaceFileW` has replacement semantics that conflict with same-id collision rejection, and Transactional NTFS is not recommended for new application designs.
+
+## Consequences
+
+The backend keeps one external contract across platforms: first append either publishes a complete log at the final name or fails without overwriting an existing log. The platform split is an implementation detail; `SessionPersistence` APIs and on-disk JSONL format do not change.
+
+Windows tests exercise the real Win32 publish path on native Windows. Power-loss behavior remains an API-contract property rather than something unit tests can prove; the testable invariants are that directory fsync is not called on Windows materialization, final-path collisions fail, temp logs are fsync'd before publication, and the resulting log loads normally.
+
+Append and repair still use ordinary file-handle fsyncs on both platforms. A failed append closes its append-only handle, reopens the log read/write, truncates it to the pre-append size, and fsyncs the rollback because Windows rejects `ftruncate` on append-only handles.