Reduce B-grade complexity hotspots in src/nncg to all-A

[tschm]

Subcategory: Code complexity — 9 → 10

Context: Surfaced by the quality scorecard in #42. No C-or-worse blocks exist (avg complexity A, 2.97), but three B-grade hotspots and two low-A-band modules keep this off a perfect score.

Files / blocks to change:

• src/nncg/inner.py:112 — _nystrom (CC 10, B)
• src/nncg/solver.py:343 — ActiveSetSolver._run (CC 9, B)
• src/nncg/inner.py:213 — Exact class / Exact.solve:247 (CC 7 / 6, B)
• Maintainability index: src/nncg/solver.py 51.84, src/nncg/inner.py 53.84 (low end of A)

Done when: every block in uvx radon cc src -s ranks A (CC ≤ 5) and uvx radon mi src -s reports all modules ≥ 65 MI.

Evidence:
```
M 343:4 ActiveSetSolver._run - B (9)
F 112:0 _nystrom - B (10)
src/nncg/solver.py - A (51.84)
```

Ref: #42