fix(sandbox): harden the per-session record and the ACL runner failure paths (review round v6)

Durable record: bound to the owning session id and validated at the fold (orphan-SID shape, temp path inside the host temp root) — a fork's copied parent record no longer provisions the child, and a tampered record fails loud. Private temp dir: random unguessable name persisted in the record, created exclusively (pre-existing entries and reparse points fail EEXIST). Persistence: a fresh provision kicks an immediate flush (no write-behind debounce), narrowing the crash window to the flush latency — documented as the one self-healing gap. Runner-failure rules: exit-gated on 127 so a confined command that prints the signature on a non-127 exit is never misclassified. Spawn: AssignProcessToJobObject failure terminates the suspended child (no hanging orphans). SandboxExecutionPolicy.sessionId is the branded SessionId. Boundary docs: qualifying clause on the absolutist sentences, NULL-DACL Known Limitation, 'full' scoped to the supported NTFS surface, CLM gate comment.

docs/core-data-structures/sandbox.md

@@ -54,12 +54,12 @@ interface SandboxExecutionPolicy {
   /** Absolute root directory `workspace-write` may write under. */
   workspaceRoot: string
   /**
-   * Opaque identity of the calling session (the `dsh-session` SessionId in
-   * string form). Backends key per-session state off it (e.g. the windows-acl
+   * Opaque identity of the calling session (the branded `dsh-session`
+   * SessionId). Backends key per-session state off it (e.g. the windows-acl
    * per-session write grant and private temp subdirectory); absent for
    * agentless calls, which fall back to per-call backend state.
    */
-  sessionId?: string
+  sessionId?: SessionId
 }
 ```