docs(spec-064): refresh stale references after monolith splits

[bug-ops]

Summary

Refreshes specs/064-durable-execution/spec.md after an SDD currency + traceability review against HEAD b15adeb9. The spec was authored before the agent-loop monolith splits (#4933, #4938) and before the implementation epic was filed, so several code anchors and planning-metadata fields had drifted. No semantics change — only references, paths, and one new caveat.

The review verdict was that the spec is substantively sound and fully traceable (all 15 FR + 14 INV + 7 NFR are covered by the child issues #4944–#4954, no orphaned IDs). These edits fix the stale references that would otherwise mislead implementors.

Changes

• Headers: **GitHub:** → epic implement native durable execution layer (zeph-durable, spec 064) #4707 + child issues feat(durable): scaffold zeph-durable crate, core newtypes, journal schema #4944–feat(sub-agents): durable promise for subagent spawn/await resume (P4) #4954 (M28); **Branch:** → per-issue feat/m28/* series.
• P1 adapter (HIGH): tier_loop.rs / execute_tier_join is in zeph-core/src/agent/tool_execution/tier_loop.rs (L1705), not zeph-agent-tools — the loop never moved to zeph-agent-tools; the monolith splits relocated it within zeph-core. Corrected in the §P1 header/body, the §Scope adapter list, the layer-placement rationale, the LLM-gate note, and §Three Hard Problems.
• P2 adapter: plan.rs / handle_plan_resume_as_string is in zeph-core (def L1006, dispatch L1161) → resume_loaded_graph (L935) → DagScheduler::resume_from at zeph-orchestration/src/scheduler/mod.rs:380.
• P3 precedent: JobStore::init line ref 136 → 137.
• NFR-DE-01: benchmark harness is per-crate crates/zeph-durable/benches/ (workspace convention), not zeph-bench (the agent-eval harness).
• spec-039 reconciliation: reference zeph-common::TaskSupervisor (the unified JoinSet wrapper) instead of a bare JoinSet.
• Persistence Schema: added a caveat documenting the pre-existing sqlite/postgres migration-sequence parity divergence (sqlite=96 vs postgres=95, numbering drifts from 075), tracked in zeph-db sqlite/postgres migration sequences diverge (file count + content parity) #4957 — it blocks the durable migration numbering in C1 (feat(durable): scaffold zeph-durable crate, core newtypes, journal schema #4944).

Verification

Docs-only change (single markdown file, no Rust code or manifests touched) — cargo fmt/clippy/test gates are not applicable. Diff: 1 file changed, +26 / −15.

Related

• Epic implement native durable execution layer (zeph-durable, spec 064) #4707 (native durable execution layer)
• zeph-db sqlite/postgres migration sequences diverge (file count + content parity) #4957 (migration parity divergence, filed alongside this review)
• Full review report: .local/handoff/sdd-review-spec-064.md