Story decomposition can leave cross-subsystem integration glue unowned, surfacing only in implementation

## Summary
When per-subsystem stories each defer their integration seam to "a later story" via `deferred:integration` markers, the actual wiring — replacing a request-dispatch/composition stub with the fully composed pipeline — may end up owned by NO story. This orphaned seam surfaces during Phase 3 implementation, forcing a mid-wave remediation story and a re-opened Phase-2 convergence.

## Proposed remediation
The story-decomposition gate (Phase 2) should verify:
1. Every `deferred:integration` marker names an owning story that actually exists.
2. Each composition/wiring seam between subsystems is explicitly story-owned before Phase 2 closes.

## Impact
An unowned seam is invisible until code lands on it, at which point the pipeline must synthesize a remediation story mid-wave and re-run Phase-2 adversarial convergence — expensive and out-of-phase.