Add inference-time model_config overrides with a sampling-key allowlist (gitea #87)

giant predict/rollout rebuilt models straight from ckpt["model_config"] with
no way to change sampling-only keys (e.g. stage2_model.n_sec.stop_sampling)
without retraining. Adds config_overrides to load_for_inference, validated
against giant.config.INFERENCE_OVERRIDES so a typo or shape-bearing key
raises CheckpointCompatibilityError up front instead of an opaque
load_state_dict mismatch. Wired as a repeatable --set dotted.path=value on
both CLI commands, recorded in the rollout YAML sidecar, and surfaced in
`giant model summary`'s output.

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

tests/test_cli_predict.py

@@ -172,3 +172,43 @@ def test_predict_exits_1_on_checkpoint_missing_model_config(tmp_path):
 
     assert result.exit_code == 1
     assert "checkpoint has no model_config" in result.output
+
+
+# ---------------------------------------------------------------------------
+# --set (gitea #87)
+# ---------------------------------------------------------------------------
+
+
+def test_predict_set_flag_without_equals_exits_1(tmp_path):
+    checkpoint = tmp_path / "missing.pt"
+
+    result = runner.invoke(
+        app,
+        ["predict", "dummy.parquet", "--checkpoint", str(checkpoint), "--set", "stop_sampling"],
+    )
+
+    assert result.exit_code == 1
+    assert "must be 'dotted.path=value'" in result.output
+
+
+def test_predict_set_flag_disallowed_path_surfaces_compat_error(tmp_path):
+    checkpoint = tmp_path / "ckpt.pt"
+    torch.save(
+        {"model_config": {"stage1_model": {}, "stage2_model": {}}, "sec_decoder": {}, "normalizer": {"sec_phys": {}}},
+        checkpoint,
+    )
+
+    result = runner.invoke(
+        app,
+        [
+            "predict",
+            "dummy.parquet",
+            "--checkpoint",
+            str(checkpoint),
+            "--set",
+            "stage1_model.hidden_dim=999",
+        ],
+    )
+
+    assert result.exit_code == 1
+    assert "not an inference-safe override" in result.output