Task: BT subgraph title position

[dnvt]

Summary: BT titled subgraphs currently render the title near the top of the subgraph instead of anchoring it at the bottom for bottom-to-top flow.

Problem: In graph BT, titles like Data Sources and Handler Group stay near the top/title row. This conflicts with directional reading and contributes to BT title-row corruption pressure.

Desired behavior:

• BT titled subgraphs place the title at a bottom-anchored position.
• Routing, repair, and title-protection logic agree on the new BT title location.
• TD, LR, and RL behavior remains unchanged.

Likely touchpoints:

• src/render/mod.rs
• src/render/repair.rs
• src/render/edge.rs
• BT-focused tests and fixtures

Plan: see planning/BT_SUBGRAPH_TITLE_POSITION.md.