artifacts: promote judge_graded + tf-margin to eval/ library (Phase 0a)

Overview / Motivation

Phase 0a of the unified persona-vectors-style artifact factory (approved plan: /home/thomasjiralerspong/.claude/plans/help-me-to-devise-vectorized-tower.md). Promote two proven-but-scripts/-resident helpers into the importable eval/ library so the unified data-generation + evaluation pipeline (later Phase-0 tasks) can import them.

Goal

Two new importable library modules, semantically identical to their sources:

• src/explore_persona_space/eval/graded_judge.py::judge_graded(items, rubric, *, n_draws, judge_model, ...) -> JudgeResult — lifted from scripts/issue778_lib.py::judge_graded: graded 0-100, multi-draw mean, DROP-never-coerce on REFUSAL / non-numeric / out-of-[0,100] returns (from BOTH arms), per-arm dropped counts reported. Internally routes through the existing eval/batch_judge.judge_completions_batch → eval/judge_dispatch stack (do NOT reimplement dispatch).
• src/explore_persona_space/eval/margin.py::compute_tf_margin(...) — the teacher-forced FIXED positive-vs-negative completion margin (the non-saturating continuous companion DV; llm-judging.md §E2 rule 19). Lift from the #722 implementation — GREP FIRST for the existing code (grep -rn 'tf_margin\|compute_tf_margin\|fixed.*margin' src/ scripts/ eval_results/issue_722/); reuse it, don't rewrite.

Scope / constraints

• Library becomes canonical; scripts/issue778_lib.py re-imports judge_graded from eval/graded_judge.py (rewire-then-dedupe per code-style.md supersede rule — no two live copies).
• Judge model pin claude-sonnet-4-5-20250929 preserved (workflow_lint.py --check-judge-model-pins passes).
• CPU unit tests (no GPU): judge_graded drops (not coerces) a REFUSAL / malformed / out-of-range return from both arms + reports per-arm dropped counts; multi-draw mean aggregation; compute_tf_margin returns the fixed pos/neg mean-logP margin on a tiny synthetic pool (mock the model forward).
• uv run ruff check/format + uv run pytest on the new tests green.

Rules to read

.claude/rules/llm-judging.md (§E2 rules 19-20 — the companion DV; drop-never-coerce), .claude/rules/marker-leakage-measurement.md (the marker DV is a DIFFERENT object — do not conflate), .claude/rules/code-style.md (supersede → dedupe).