Add calibrated harder needle probe

[Danny1218]

Design a harder long-context or retrieval-style needle probe that still has a usable FP16 baseline.

Goal:

• Probe whether quantization fragility appears in context-depth stress without relying on a ceiling baseline.

Acceptance criteria:

• Add probe generation and validation code without mutating frozen probe sets.
• Verify FP16 calibration before using it in headline comparisons.
• Document scorer limitations and avoid claims from weak/floor/ceiling baselines.