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/src/index.ts

@@ -15,6 +15,7 @@ import { isJsonValue } from './json.ts'
 
 export * from './types.ts'
 export { isJsonValue } from './json.ts'
+export { interruptedTurnClosers } from './repair.ts'
 
 declare module 'cordis' {
   interface Context {