Implement stage2_model.stage1_context = "sampled" (gitea #41)

Stage 2 was trained on ground-truth stage-1 outcomes but deployed on
sampled ones, and in a rollout that gap compounds over every step of
every track — the same train/inference gap teacher_forcing="scheduled"
already closes within stage 2, just never applied at the stage
boundary. "sampled" was declared in the schema but rejected loudly by
validate_config as unimplemented; this lands the real implementation.

Mirrors the existing scheduled-sampling precedent rather than a hard
switch: new stage2_model.ctx_p_start/ctx_p_end (defaults 1.0 -> 0.0)
linearly ramp P(condition on ground truth) from epoch 0 to the final
epoch, so stage 2 doesn't chase a wildly moving stage-1 target early in
training. Per the plan discussed with the user: the sample is drawn
from stage 1's sampling_model() (EMA weights when present, matching
what inference actually deploys), mixed per example via a Bernoulli
draw (never blended within a row), and validation always uses the
ground truth regardless of the schedule. Fixes a latent bug the same
pattern would otherwise have hit: every sampler in giant/sample.py
flips its model to .eval() with no restore, so sampling from the raw
(non-EMA) stage-1 model mid-step now explicitly restores its .training
flag afterward to avoid silently corrupting stage 1's own training mode
for the rest of the epoch.

validate_config now enforces stage1_context in {"truth", "sampled"},
requires both stages active for "sampled" (nothing to sample from
otherwise), range-checks ctx_p_start/ctx_p_end, and rejects the
ctx_p_start = ctx_p_end = 1.0 configuration as an unadvertised no-op
identical to "truth".

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_config.py

@@ -780,13 +780,79 @@ def test_validate_config_bad_stop_sampling_rejected():
         assert "stop_sampling" in str(e)
 
 
-def test_validate_config_stage1_context_sampled_not_implemented():
+def test_validate_config_stage1_context_sampled_accepted_with_both_stages_active():
+    """gitea #41: 'sampled' is now implemented, so DEFAULT_CONFIG's
+    stage1_model/stage2_model.active = true (both) must let it through."""
     cfg = _cfg_with(**{"stage2_model.stage1_context": "sampled"})
+    gconfig.validate_config(cfg)  # must not raise
+
+
+def test_validate_config_bad_stage1_context_rejected():
+    cfg = _cfg_with(**{"stage2_model.stage1_context": "bogus"})
     try:
         gconfig.validate_config(cfg)
         assert False, "expected ValueError"
     except ValueError as e:
-        assert "sampled" in str(e)
+        assert "stage1_context" in str(e)
+
+
+def test_validate_config_stage1_context_sampled_requires_stage1_active():
+    cfg = _cfg_with(
+        **{
+            "stage2_model.stage1_context": "sampled",
+            "stage1_model.active": False,
+        }
+    )
+    try:
+        gconfig.validate_config(cfg)
+        assert False, "expected ValueError"
+    except ValueError as e:
+        assert "sampled" in str(e) and "stage1_model.active" in str(e)
+
+
+def test_validate_config_stage1_context_sampled_requires_stage2_active():
+    cfg = _cfg_with(
+        **{
+            "stage2_model.stage1_context": "sampled",
+            "stage2_model.active": False,
+        }
+    )
+    try:
+        gconfig.validate_config(cfg)
+        assert False, "expected ValueError"
+    except ValueError as e:
+        assert "sampled" in str(e) and "stage2_model.active" in str(e)
+
+
+@pytest.mark.parametrize("key", ["ctx_p_start", "ctx_p_end"])
+@pytest.mark.parametrize("value", [-0.1, 1.1])
+def test_validate_config_ctx_p_out_of_range_rejected(key, value):
+    cfg = _cfg_with(
+        **{
+            "stage2_model.stage1_context": "sampled",
+            f"stage2_model.{key}": value,
+        }
+    )
+    try:
+        gconfig.validate_config(cfg)
+        assert False, "expected ValueError"
+    except ValueError as e:
+        assert key in str(e)
+
+
+def test_validate_config_stage1_context_sampled_always_truth_rejected_as_noop():
+    cfg = _cfg_with(
+        **{
+            "stage2_model.stage1_context": "sampled",
+            "stage2_model.ctx_p_start": 1.0,
+            "stage2_model.ctx_p_end": 1.0,
+        }
+    )
+    try:
+        gconfig.validate_config(cfg)
+        assert False, "expected ValueError"
+    except ValueError as e:
+        assert "ctx_p_start" in str(e) and "ctx_p_end" in str(e)
 
 
 def test_validate_config_n_sec_truth_rejected_for_rollout_capable_checkpoint():