RAG Phase 5: 2026 refresh + article-aware chunking (eval-gated)

[VforVitorio]

Sub of Epic #318. documents/audits/AUDIT_RAG_LAYER.md Phase 5 (size M-L). Eval-gated on Phase 3.

Scope: 2026 refresh + article-aware chunking.

• Ingest the 2026 FIA PDFs (the ingest side of 2026-reg F-10; coordinate with Epic: Audit 2026-Reg / concept-drift readiness #189, do not duplicate).
• A/B article-aware chunking to fix the first-regex-match off-by-one article tagging and the wasteful 512-char chunks (bge-m3 accepts 8192), gated on the Phase-3 eval so the change is proven to help.

Deliverable: a 2026-capable, article-correct index whose chunking change beats the baseline on the Phase-3 eval.