Make default checkpoint out_dir name reflect only non-default hyperparams

Previously the same fixed 7 fields (mode/hidden_dim/n_blocks/emb_dim/
conditioning/lr/batch_size) were always baked into the name, even for a
vanilla run, and router config wasn't represented at all. Now
default_out_dir_name only includes fields that differ from
DEFAULT_CONFIG, adds router/seed/epochs as candidates, and caps at 6
shown fields with a hashed overflow suffix for heavily-swept configs.

tests/test_config.py

@@ -1,3 +1,5 @@
+from datetime import datetime
+
 from giant import config as gconfig
 
 
@@ -144,4 +146,101 @@ def test_resolve_expert_dims_explicit_override_wins():
 def test_resolve_expert_dims_partial_override_mixes_explicit_and_inherited():
     router_cfg = {"expert_hidden_dim": 128, "expert_n_blocks": 0}
     hidden_dim, n_blocks = gconfig.resolve_expert_dims(router_cfg, 512, 6)
-    assert (hidden_dim, n_blocks) == (128, 6)  # n_blocks inherited, hidden_dim not
+    assert (hidden_dim, n_blocks) == (128, 6)
+
+
+def _default_cfg(**overrides):
+    train_overrides = {
+        k: v for k, v in overrides.items() if k in gconfig.DEFAULT_CONFIG["train"]
+    }
+    model_overrides = {
+        k: v for k, v in overrides.items() if k in gconfig.DEFAULT_CONFIG["model"]
+    }
+    router_overrides = overrides.get("router")
+    if router_overrides:
+        model_overrides["router"] = router_overrides
+    return gconfig.merge_cli_overrides(
+        gconfig.DEFAULT_CONFIG, None, train_overrides, model_overrides
+    )
+
+
+_NOW = datetime(2026, 7, 29, 14, 30)
+
+
+def test_default_out_dir_name_all_defaults_is_just_the_timestamp():
+    cfg = _default_cfg()
+    assert gconfig.default_out_dir_name(cfg, now=_NOW) == "20260729_1430"
+
+
+def test_default_out_dir_name_single_non_default_field():
+    cfg = _default_cfg(hidden_dim=512)
+    assert gconfig.default_out_dir_name(cfg, now=_NOW) == "20260729_1430_h512"
+
+
+def test_default_out_dir_name_conditioning_embedding_shown_abbreviated():
+    cfg = _default_cfg(conditioning="embedding")
+    assert gconfig.default_out_dir_name(cfg, now=_NOW) == "20260729_1430_cemb"
+
+
+def test_default_out_dir_name_conditioning_default_omitted():
+    cfg = _default_cfg(conditioning="physical")
+    assert gconfig.default_out_dir_name(cfg, now=_NOW) == "20260729_1430"
+
+
+def test_default_out_dir_name_router_enabled_shown_as_unit():
+    cfg = _default_cfg(router={"enabled": True, "type": "energy", "n_experts": 8})
+    assert gconfig.default_out_dir_name(cfg, now=_NOW) == "20260729_1430_r-energy8"
+
+
+def test_default_out_dir_name_router_disabled_omitted_even_if_subfields_nondefault():
+    cfg = _default_cfg(router={"enabled": False, "type": "pdg", "n_experts": 8})
+    assert gconfig.default_out_dir_name(cfg, now=_NOW) == "20260729_1430"
+
+
+def test_default_out_dir_name_mode_shown_bare_no_prefix():
+    cfg = _default_cfg(mode="wgan")
+    assert gconfig.default_out_dir_name(cfg, now=_NOW) == "20260729_1430_wgan"
+
+
+def test_default_out_dir_name_overflow_caps_and_hashes_remainder():
+    cfg = _default_cfg(
+        mode="wgan",
+        router={"enabled": True, "type": "energy", "n_experts": 8},
+        conditioning="embedding",
+        hidden_dim=512,
+        n_blocks=8,
+        emb_dim=32,
+        lr=1e-3,
+        batch_size=2048,
+        seed=3,
+        epochs=200,
+    )
+    name = gconfig.default_out_dir_name(cfg, now=_NOW)
+    # First 6 by priority: mode, router, conditioning, hidden_dim, n_blocks, emb_dim.
+    assert name.startswith("20260729_1430_wgan_r-energy8_cemb_h512_b8_e32_+4more-")
+    digest = name.split("-")[-1]
+    assert len(digest) == 6
+
+
+def test_default_out_dir_name_overflow_hash_is_deterministic_and_value_sensitive():
+    base = dict(
+        mode="wgan",
+        router={"enabled": True, "type": "energy", "n_experts": 8},
+        conditioning="embedding",
+        hidden_dim=512,
+        n_blocks=8,
+        emb_dim=32,
+        lr=1e-3,
+        batch_size=2048,
+        seed=3,
+        epochs=200,
+    )
+    name_a = gconfig.default_out_dir_name(_default_cfg(**base), now=_NOW)
+    name_b = gconfig.default_out_dir_name(_default_cfg(**base), now=_NOW)
+    assert name_a == name_b  # stable across calls with the same overflow set
+
+    changed = dict(base, epochs=999)
+    name_c = gconfig.default_out_dir_name(_default_cfg(**changed), now=_NOW)
+    assert (
+        name_c != name_a
+    )  # differs when an overflowed value changes  # n_blocks inherited, hidden_dim not