feat(session-persistence): preserve interrupted turns on crash; don't truncate (review #33)

A crash can leave a durable log whose final turn never closed. The old
behavior truncated everything after the last turn/end as a "crash tail".
But a single turn can be HUGE in a long-horizon task (many steps, large
tool output), so truncating it silently destroys real, durably-written
work — truncating a turn is wrong.

New crash recovery (ADR 0018): load() PRESERVES the interrupted turn's
events and CLOSES the orphaned turn by durably appending synthetic
boundary events — a step/end if a step was open, then a turn/end carrying
the new merge-extensible TurnEndReason {kind:'interrupted'}. load()
returns the balanced log, so a resumed session is immediately usable. Only
a never-fully-written TORN tail fragment is discarded; corruption in the
committed region is still unloadable.

- dsh-session: TurnEndReason {kind:'interrupted'} + shared
  interruptedTurnClosers() repair helper.
- JSONL backend: scanLog preserves the longest contiguous prefix
  (including a partial final turn); loadCore truncates a torn fragment and
  durably writes the closers, returning the balanced log.
- runPersistenceContract gains a crash-recovery test (both backends + mock).
- Docs: ADR 0018/0017, architecture.md, package READMEs.

Also (review #33): RFC 013 records the "move event vocabulary to Zod"
question (merge-extensible maps → runtime schema registry) + blast radius;
deferred, not done here.

packages/session-persistence-jsonl/README.md

@@ -24,7 +24,7 @@ The JSONL durable session-persistence backend — a concrete `SessionPersistence
 
 - **Lazy materialization.** `create(meta)` writes nothing; the `.jsonl` (header + first batch) is written atomically (temp-write + `fsync` + rename) on the first `append`. A created-but-never-appended session leaves nothing on disk and is absent from `has`/`list`.
 - **Append-only.** Committed events (at or below a flushed `turn/end`) are never rewritten. Subsequent appends are line appends at EOF + `fsync`.
-- **Truncation-repair.** `load` returns events only up to the last complete `turn/end` and records the byte offset of any never-committed crash tail; the first post-load `append` `ftruncate`s to that offset (+ `fsync`) before writing, atomically discarding only the uncommitted tail.
+- **Crash recovery — close, don't truncate.** A crash can leave a log whose final turn never closed (real events after the last `turn/end`). `load` PRESERVES those events (a turn can be huge — they are real work) and closes the orphaned turn by durably appending synthetic boundary events (a `step/end` if a step was open, then `turn/end {kind:'interrupted'}`), returning a balanced log. Only a never-fully-written **torn tail fragment** (a final line with no newline / unparseable) is `ftruncate`d away before the closers are written. See ADR 0018.
 - **Contiguous-seq.** `load` rejects a mid-log parse error or `seq` gap (unloadable); `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.
 - **Format version.** Only v1 is supported; `load` rejects an unknown version. A future format change requires a version bump + migration.