Extract predict/rollout's duplicated inference bootstrap into giant.checkpoint_io (issues.md Issue 5)

giant predict and giant rollout each carried a ~65-line, independently
drifting copy of "load checkpoint -> validate -> resolve conditioning axes
-> restore normalizers/vocab maps -> build models -> load weights", plus a
third partial copy of _conditioning_axes in analysis/router_gating.py. A
silent divergence there doesn't crash, it makes the two commands run
different physics from the same checkpoint with no test coverage anywhere
along that path.

giant/checkpoint_io.py now holds the single implementation:
load_for_inference() + an InferenceContext dataclass, raising
CheckpointCompatibilityError (verbatim message text preserved) instead of
calling typer directly, so it can be unit-tested and imported from
non-Typer code. router_gating.py's load_router imports conditioning_axes
from it lazily, keeping its "no torch at module scope" contract intact.

Adds 17 direct unit tests for load_for_inference/conditioning_axes/stage_cfg
plus CLI smoke tests confirming the error surfaces as typer.Exit(1) through
predict and rollout — previously zero coverage on this path.

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

tests/test_cli_rollout.py

@@ -0,0 +1,33 @@
+"""Thin CLI smoke coverage for `giant rollout` (issues.md Issue 5) — confirms
+the CheckpointCompatibilityError raised by giant.checkpoint_io.load_for_inference
+surfaces as a clean typer.Exit(1) with the expected message, end-to-end
+through the CLI, not just at the giant.checkpoint_io unit level."""
+
+from __future__ import annotations
+
+import torch
+from typer.testing import CliRunner
+
+from giant.cli import app
+
+runner = CliRunner()
+
+
+def test_rollout_exits_1_on_checkpoint_missing_model_config(tmp_path):
+    checkpoint = tmp_path / "bad.pt"
+    torch.save({"sec_decoder": {}, "normalizer": {"sec_phys": {}}}, checkpoint)
+
+    result = runner.invoke(
+        app,
+        [
+            "rollout",
+            "dummy.parquet",
+            "--checkpoint",
+            str(checkpoint),
+            "--geometry",
+            "dummy_geometry.pkl",
+        ],
+    )
+
+    assert result.exit_code == 1
+    assert "checkpoint has no model_config" in result.output