feat: heuristic coordinate proposer (drafting)

Per sub-project-2 plan Task 7 / spec §7. HeuristicProposer seeds the brain plane
from archive priors and the mood plane from title/description keyword nudges,
clamps to 0..4, and emits a one-line rationale. Deterministic, no I/O, no ML
(honors design §11) — only a DRAFT a human reviews.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_drafting.py

@@ -0,0 +1,79 @@
+from hef.selection import Coordinate
+from tools.drafting import Draft, HeuristicProposer, Signals
+
+
+def _sig(**o):
+    base = dict(
+        title="",
+        description="",
+        source_archive="nasa",
+        mode="video",
+        duration_s=600,
+    )
+    base.update(o)
+    return Signals(**base)
+
+
+def test_returns_draft_with_coordinate_and_rationale():
+    d = HeuristicProposer().propose(_sig())
+    assert isinstance(d, Draft)
+    assert isinstance(d.coordinate, Coordinate)
+    assert d.rationale and "\n" not in d.rationale
+
+
+def test_librivox_seeds_left():
+    d = HeuristicProposer().propose(
+        _sig(title="Meditations", source_archive="librivox", mode="audio")
+    )
+    assert d.coordinate.left >= 3 and d.rationale
+
+
+def test_music_archives_seed_right():
+    for arch in ("musopen", "fma", "nasa", "freesound"):
+        d = HeuristicProposer().propose(_sig(source_archive=arch, mode="audio"))
+        assert d.coordinate.right >= 3, arch
+
+
+def test_internet_archive_seeds_left():
+    d = HeuristicProposer().propose(_sig(source_archive="internet_archive"))
+    assert d.coordinate.left >= 3
+
+
+def test_storm_seeds_dark():
+    d = HeuristicProposer().propose(
+        _sig(title="Thunderstorm at Night", source_archive="nasa")
+    )
+    assert d.coordinate.dark >= 2
+
+
+def test_sunrise_seeds_light():
+    d = HeuristicProposer().propose(
+        _sig(title="Sunrise over the Garden", source_archive="nasa")
+    )
+    assert d.coordinate.light >= 2
+
+
+def test_coordinates_clamped_to_0_4():
+    d = HeuristicProposer().propose(
+        _sig(
+            title="storm night war funeral decay requiem minor",
+            description="noir death grief",
+            source_archive="librivox",
+        )
+    )
+    for v in (d.coordinate.left, d.coordinate.right, d.coordinate.dark, d.coordinate.light):
+        assert 0 <= v <= 4
+
+
+def test_rationale_cites_a_signal():
+    d = HeuristicProposer().propose(
+        _sig(title="Storm", source_archive="librivox", mode="audio")
+    )
+    assert "librivox" in d.rationale.lower()
+
+
+def test_deterministic():
+    s = _sig(title="Storm at Dawn", source_archive="nasa")
+    a = HeuristicProposer().propose(s)
+    b = HeuristicProposer().propose(s)
+    assert a == b