QP: outward completion propagation for pending fragments

[bjagg]

Part of the ADR-0003 epic #1105. Plan: docs/operations/proposals/advisor-qp-direct-retrieval.md (PR #1104). Decision: ADR-0003.

Goal: Propagate completion outward when a pending fragment lands (today it's poll-over-in-memory only).

Scope:

• Extend /orchestration/results handling (query_planner_service/core.py:207-274) to propagate per-part completion outward, not just flip an in-memory flag.
• Consumer-facing mechanism — poll recommended for the internal QP↔semantic-search hop (user-facing push is handled in I).

Size: L · Depends on: F, G