Merge remote-tracking branch 'origin/master' into worktree/semantic-session-checkpoints

# Conflicts:
#	packages/session-persistence/session-persistence-jsonl/README.md

packages/session-persistence/session-persistence-jsonl/README.md

@@ -31,7 +31,7 @@ A root belongs to one encoding. Startup discovery and targeted lookup reject the
 
 ## Durability and crash semantics
 
-- **Lazy materialization.** `create(meta)` writes nothing; on the first `append`, the backend writes and `fsync`s a temporary file, publishes it without overwrite via a hard link, then `fsync`s the directory when the host supports it. A created-but-never-appended session leaves nothing on disk and is absent from `list`.
+- **Lazy materialization.** `create(meta)` writes nothing; on the first `append`, the backend writes and `fsync`s the encoded header and first batch in a temporary file. POSIX publishes it without overwrite via a hard link and `fsync`s the parent directory. Windows publishes it without overwrite via `MoveFileExW(..., MOVEFILE_WRITE_THROUGH)` and creates missing directories through the same write-through pattern. A created-but-never-appended session leaves nothing on disk and is absent from `list`.
 - **Append-only.** Flushed events are never rewritten. Subsequent raw batches append lines; compressed batches append one frame. Both paths `fsync`, and a caught write or sync failure rolls the file back to its prior byte length.
 - **Crash recovery — preserve valid tail work.** `load` validates every complete compressed frame and scans their decompressed JSONL. If the last frame is structurally incomplete, the reader keeps its complete decoded records, truncates from that frame's start, and re-encodes those records with the synthetic tool, step, and turn closers required by the shared [persistence contract](../../../.agents/notes/implemented/architecture/2026-06-14-session-persistence.md). Raw mode truncates from its first incomplete line. A checksum/decompression failure in a complete frame, or a defect at or before the last committed `turn/end`, is corruption and rejects.
 - **Contiguous-seq.** `append` rejects a batch whose first `seq` does not continue the stored log, and rejects non-JSON-serializable `event.data` naming the offending event type.
@@ -62,5 +62,4 @@ JSONL storage does not mutate live request prefixes. A resumed loop can reuse pr
 - **Compressed files are not directly line-readable** — use the backend to load them, or select `compression: 'none'` before writing a fresh root when text fixtures or external line readers are required.
 - **Nothing deletes session files** — logs accumulate under `root` until removed externally (the seam has no deletion surface).
 - **Single-process assumption** — per-session serialization and the write cursor live in this process; two processes appending to the same `root` are not coordinated.
-- **Initial materialization requires hard-link support** — first append uses `link()` so same-id races fail instead of overwriting a committed log; a filesystem that cannot create hard links cannot host this backend.
-- **Windows cannot `fsync` directory handles through Node** — the backend tolerates only Windows `EPERM` from directory `fsync`; file-content `fsync` remains mandatory, but a crash can lose a newly published directory entry on a host without an equivalent directory-sync primitive.
+- **POSIX materialization requires hard-link support** — first append uses `link()` so same-id races fail instead of overwriting a committed log; Windows uses write-through rename without replacement.