feat(figment): add render_figment_overlay() to layers.py

Implements phase-aware (REVEAL/HOLD/DISSOLVE) ANSI cursor-positioning overlay
renderer for figment glyphs, with deterministic shuffle seeding and gradient
coloring via _color_codes_to_ansi(). Includes 6 TDD tests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_figment_overlay.py

@@ -0,0 +1,60 @@
+"""Tests for render_figment_overlay in engine.layers."""
+
+from effects_plugins.figment import FigmentPhase, FigmentState
+from engine.layers import render_figment_overlay
+
+
+def _make_state(phase=FigmentPhase.HOLD, progress=0.5):
+    return FigmentState(
+        phase=phase,
+        progress=progress,
+        rows=["█▀▄ █", " ▄█▀ ", "█   █"],
+        gradient=[46, 40, 34, 28, 22, 22, 34, 40, 46, 82, 118, 231],
+        center_row=10,
+        center_col=37,
+    )
+
+
+class TestRenderFigmentOverlay:
+    def test_returns_list_of_strings(self):
+        state = _make_state()
+        result = render_figment_overlay(state, 80, 24)
+        assert isinstance(result, list)
+        assert all(isinstance(s, str) for s in result)
+
+    def test_contains_ansi_positioning(self):
+        state = _make_state()
+        result = render_figment_overlay(state, 80, 24)
+        # Should contain cursor positioning escape codes
+        assert any("\033[" in s for s in result)
+
+    def test_reveal_phase_partial(self):
+        state = _make_state(phase=FigmentPhase.REVEAL, progress=0.0)
+        result = render_figment_overlay(state, 80, 24)
+        # At progress 0.0, very few cells should be visible
+        # Result should still be a valid list
+        assert isinstance(result, list)
+
+    def test_hold_phase_full(self):
+        state = _make_state(phase=FigmentPhase.HOLD, progress=0.5)
+        result = render_figment_overlay(state, 80, 24)
+        # During hold, content should be present
+        assert len(result) > 0
+
+    def test_dissolve_phase(self):
+        state = _make_state(phase=FigmentPhase.DISSOLVE, progress=0.9)
+        result = render_figment_overlay(state, 80, 24)
+        # At high dissolve progress, most cells are gone
+        assert isinstance(result, list)
+
+    def test_empty_rows(self):
+        state = FigmentState(
+            phase=FigmentPhase.HOLD,
+            progress=0.5,
+            rows=[],
+            gradient=[46] * 12,
+            center_row=0,
+            center_col=0,
+        )
+        result = render_figment_overlay(state, 80, 24)
+        assert result == []