[Proposed] Persona representation across pipeline: base → midtrain → post-train → post-EM

From EXPERIMENT_QUEUE.md, added 2026-04-16

Measure how the persona representation (persona vectors, assistant axis, persona separability, identity markers) evolves at each stage of the standard pipeline.

Checkpoints to probe:

1. base Qwen-2.5-7B
2. post-coupling SFT
3. post-midtrain (Tulu SFT 25%)
4. post-post-train (Tulu DPO)
5. post-EM (LoRA)

Metrics: per-layer persona separability (LDA accuracy on 20-persona grid), persona vector norms, cosine(persona_i, persona_j) matrix, assistant-axis alignment, capability-direction alignment.

Key questions:

• (a) Where in the pipeline does the EM-relevant persona structure first appear?
• (b) Does DPO preserve/alter persona geometry vs SFT?
• (c) Does EM primarily warp persona directions or their capability entanglement?

Reuses existing checkpoints from Aim 5 25% midtrain matrix (5 conditions × 4 checkpoints each already on HF Hub).

Compute: ~4-6 GPU-hours (activation extraction across 4-5 checkpoints × 20 personas × 928 prompts, layers 10-25). No training.

Gate-keeper priority: HIGH (directly answers "when does EM susceptibility emerge" — foundational for the defense story).