Reject stage2_model.stage1_context = 'sampled' as unimplemented (issues.md Issue 1)

trainers.py unconditionally trains stage 2 against the ground-truth
stage-1 output (stage1_ctx = x1_s1.detach()), but 'sampled' was accepted
by validate_config, stored in config.toml and the checkpoint's
model_config, and silently trained identically to 'truth' — mislabeling
every downstream artifact for a run launched with
--stage2-stage1-context sampled. Mirrors the existing stop_token
validate_config pattern. User chose the immediate fix (reject loudly)
over the proper fix (actually implement sampled context), which is
scoped to Issue 16.

Also updates the _KNOWN_UNUSED reason for stage2_model.stage1_context
(added by Issue 5's consumed-keys audit) to reflect that the value is
now rejected rather than silently accepted, and drops the now-invalid
--stage2-stage1-context sampled case from test_stage2_only_knobs (a
full CLI invocation) — that flag's plumbing is still covered at the
overrides-dict level by test_overrides_from_flags_stage2_only_knobs.

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

tests/test_cli_train_overrides.py

@@ -41,6 +41,10 @@ def test_stage_prefixed_generator_overrides_shared_mode(monkeypatch, tmp_path):
 
 
 def test_stage2_only_knobs(monkeypatch, tmp_path):
+    # --stage2-stage1-context is exercised separately at the overrides-dict
+    # level (test_overrides_from_flags_stage2_only_knobs in test_config.py):
+    # its only non-default value, "sampled", is rejected by validate_config
+    # (issues.md Issue 1), so it can't appear in a full CLI invocation here.
     cfg = _invoke_and_capture_cfg(
         monkeypatch,
         tmp_path,
@@ -53,15 +57,12 @@ def test_stage2_only_knobs(monkeypatch, tmp_path):
             "32",
             "--stage2-context-dim",
             "16",
-            "--stage2-stage1-context",
-            "sampled",
         ],
     )
     assert cfg["stage2_model"]["decoder"] == "one_shot"
     assert cfg["stage2_model"]["k_max"] == 8
     assert cfg["stage2_model"]["hidden_dim"] == 32
     assert cfg["stage2_model"]["context_dim"] == 16
-    assert cfg["stage2_model"]["stage1_context"] == "sampled"
     # untouched stage1 defaults
     assert cfg["stage1_model"]["hidden_dim"] == 256