refactor: named pairset JSONs + explicit --vhack-pairs-path, remove None fallback

- scripts/pairset_build_authored.py: exports pairs.py::PAIRS to out/pairsets/pairs_authored.json
- scripts/pairset_build_progsets.py: copy of attic/make_pairsets.py under new naming convention
- out/pairsets/pairs_authored.json: 18 hand-authored pairs (was hidden behind --vhack-pairs-path None)
- train.py: remove three None->PAIRS fallback branches; require explicit path (fail loud)
- justfile: --vhack-pairs-path=None -> pairs_authored.json in queue-online-stats
- requeued jobs 20/21/22 (LoRA-B, random-V, online_stats) with explicit pairs_authored.json

Co-Authored-By: Claudypoo <288921227+claudypoo@users.noreply.github.com>

scripts/pairset_build_authored.py

@@ -0,0 +1,21 @@
+"""Export the hand-authored HackPairs from src/vgrout/pairs.py to a named JSON.
+
+These are 18 hand-crafted (hack, clean) pairs where both completions share the
+same solution body but differ only in whether run_tests() uses real assert vs
+fake/commented tests. They are the smallest, highest-precision pairset.
+
+Run:
+    uv run python scripts/pairset_build_authored.py
+Output:
+    out/pairsets/pairs_authored.json
+"""
+import json
+from dataclasses import asdict
+from pathlib import Path
+
+from vgrout.pairs import PAIRS
+
+out = Path("out/pairsets/pairs_authored.json")
+out.parent.mkdir(parents=True, exist_ok=True)
+out.write_text(json.dumps([asdict(p) for p in PAIRS]))
+print(f"wrote {len(PAIRS)} pairs -> {out}")