Maintenance release. The HiggsField integration is now built on the
official higgsfield-client Python SDK (https://github.com/higgsfield-ai/higgsfield-client)
rather than a reverse-engineered REST surface.

Highlights:
- Native HiggsField via the official SDK, no longer speculative
- Credentials: HF_API_KEY + HF_API_SECRET (or combined HF_KEY)
- Legacy HIGGSFIELD_API_KEY_ID/_SECRET still accepted for backwards compat
- Default model: seedance_2_0 (from the published model catalog)
- Replicate fallback unchanged
- Test isolation hardened across the suite — 268 passing

Removed at HiggsField's request:
- docs/higgsfield-api-notes.md
- scripts/analyse_higgsfield_har.py

Going forward, the supported integration path is the SDK against
platform.higgsfield.ai, and the MCP endpoint at mcp.higgsfield.ai/mcp.