feat(12): load training data from a raw dir, a curated jsonl, or the Hub

iter_keepers read only raw per-trajectory .json -- one of three input shapes.
Add load_turns(data, hub, revision) yielding assistant-bearing turns from a raw
rollout dir (keep-filtered), a curated .jsonl corpus (one record per line), or
the published corpus via posix-sdc's load_trajectories (the in-repo data/ of a
checkout, else the Hugging Face Hub). sekft-train gains --hub and --revision and
dispatches --data by dir-vs-.jsonl; train() and inspect() use it.

Raw-rollout reading stays sekft-local; curated + Hub reuse posix-sdc's loader,
imported lazily so the raw/jsonl paths need neither posix-sdc nor huggingface_hub
installed. Unit tests cover the raw-dir and jsonl dispatch.

tests/unit/test_load.py

@@ -0,0 +1,35 @@
+"""Unit tests for the trainer's three-source data loader (raw dir / curated
+jsonl). The Hub path delegates to posix-sdc and is covered there."""
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+import pytest
+
+from tiararodney.sekft import sft
+
+
+def test_load_turns_from_raw_dir(tmp_path: Path) -> None:
+    (tmp_path / "a.json").write_text(json.dumps(
+        {"keep": True, "turns": [{"role": "assistant", "content": "ls"}]}))
+    (tmp_path / "b.json").write_text(json.dumps(   # not kept -> excluded
+        {"keep": False, "turns": [{"role": "assistant", "content": "rm -rf /"}]}))
+    got = list(sft.load_turns(tmp_path))
+    assert len(got) == 1
+    assert got[0][0]["content"] == "ls"
+
+
+def test_load_turns_from_jsonl(tmp_path: Path) -> None:
+    f = tmp_path / "corpus.jsonl"
+    f.write_text("\n".join(json.dumps({"turns": [{"role": "assistant", "content": c}]})
+                           for c in ("ls", "cat x")) + "\n")
+    got = list(sft.load_turns(f))
+    assert [t[0]["content"] for t in got] == ["ls", "cat x"]
+
+
+def test_load_turns_rejects_other_paths(tmp_path: Path) -> None:
+    bad = tmp_path / "notes.txt"
+    bad.write_text("hi")
+    with pytest.raises(SystemExit):
+        list(sft.load_turns(bad))