artifacts: promote contrastive negatives to artifacts/negatives.py (Phase 0c)

Overview / Motivation

Phase 0c of the unified artifact factory (plan: /home/thomasjiralerspong/.claude/plans/help-me-to-devise-vectorized-tower.md). The artifacts/ package is now on main (behavior.py, context.py from #852). Promote the contrastive-negatives machinery into the package as a reusable panel registry.

Goal

src/explore_persona_space/artifacts/negatives.py:

• NegativeContext (promoted from scripts/issue664_common.py) — interoperating with the new artifacts/context.Context resolver where sensible (a negative is a context under which B is NOT trained).
• A panel registry (named negative panels) + a default_panel() seeded from the issue664 4-context panel (police / PersonaHub / rephrase-curious / wildchat-tech-support), always including the bare default assistant as a negative (the safety target; the highest-value negative, #464).
• assert_panel_disjoint_from_sources(realized_sources) — the HARD invariant (panel ∩ realized sources == ∅, at slug AND identity level, #527/#538), run at build time against the REALIZED panel.
• A ~1:1 positives-to-total-negatives interleave helper (negatives split across the panel).

Scope / constraints

• Reuse the issue664_common implementation; make artifacts/negatives.py canonical and have issue664_common re-import from it (supersede→dedupe per code-style.md — no two live copies).
• Append any package export to artifacts/__init__.py append-only (sibling Phase-0 tasks 0e/0f land concurrently — minimize __init__.py merge conflict).
• CPU unit tests: disjointness assert FIRES on a source∩panel overlap (slug + identity); default assistant always present; ratio helper splits ~1:1.
• uv run ruff check/format + uv run pytest green.

Rules to read

.claude/rules/contrastive-negatives.md, .claude/rules/code-style.md (supersede→dedupe).