Improve arch diff dtype reporting and Gemma4 test coverage (#273)

## Changes

### Graph diff improvements (`_graph_diff.py`)
- **Interface change details**: Show dtype and shape specifics instead
of generic 'input[N] changed'. Example: `input[3]: dtype BOOL → FLOAT;
shape [?, 1, ?, ?] → [?, ?]`
- **Initializer dtype distribution**: Show per-dtype count changes
instead of just total count. Example: `FLOAT16: 42 → 38, FLOAT: 0 → 4` —
makes it easy to spot when a PR changes the compute precision of
weights.

### Test config improvements (`_test_configs.py`)
- Add `sliding_window=8` to both `gemma4_text` and `gemma4` multimodal
test configs. This exercises the sliding window mask generation path in
L1 tests, which was previously untested by the parametrized suite.

### Testing
- 2724 tests pass, 0 failures
- All 15 Gemma4 tests pass
- All 27 graph diff tests pass

Signed-off-by: Justin Chu <justinchu@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: justinchuby

src/mobius/_graph_diff.py

@@ -191,6 +191,18 @@ def canonicalize_graph(graph: ir.Graph) -> dict:
 # =====================================================================
 
 
+def _describe_port_diff(base_port: dict, head_port: dict) -> str:
+    """Describe what changed between two I/O port dicts (dtype, shape)."""
+    parts: list[str] = []
+    bd, hd = base_port.get("dtype", "?"), head_port.get("dtype", "?")
+    if bd != hd:
+        parts.append(f"dtype {bd} → {hd}")
+    bs, hs = base_port.get("shape", []), head_port.get("shape", [])
+    if bs != hs:
+        parts.append(f"shape {bs} → {hs}")
+    return "; ".join(parts) or "changed"
+
+
 def diff_graphs(base: dict, head: dict) -> list[dict[str, Any]]:
     """Compare two canonical graph representations.
 
@@ -217,10 +229,12 @@ def diff_graphs(base: dict, head: dict) -> list[dict[str, Any]]:
         # Check for dtype/shape changes on matched ports
         for idx, (bv, hv) in enumerate(zip(b_in, h_in)):
             if bv != hv:
-                details_parts.append(f"input[{idx}] changed")
+                diffs = _describe_port_diff(bv, hv)
+                details_parts.append(f"input[{idx}]: {diffs}")
         for idx, (bv, hv) in enumerate(zip(b_out, h_out)):
             if bv != hv:
-                details_parts.append(f"output[{idx}] changed")
+                diffs = _describe_port_diff(bv, hv)
+                details_parts.append(f"output[{idx}]: {diffs}")
         changes.append(
             {
                 "type": "interface_change",
@@ -232,10 +246,26 @@ def diff_graphs(base: dict, head: dict) -> list[dict[str, Any]]:
     b_inits = base.get("initializers", [])
     h_inits = head.get("initializers", [])
     if b_inits != h_inits:
+        details_parts_init: list[str] = []
+        if len(b_inits) != len(h_inits):
+            details_parts_init.append(f"count {len(b_inits)} → {len(h_inits)}")
+        # Summarize dtype distribution changes
+        from collections import Counter
+
+        b_dtypes = Counter(i["dtype"] for i in b_inits)
+        h_dtypes = Counter(i["dtype"] for i in h_inits)
+        if b_dtypes != h_dtypes:
+            dtype_parts = []
+            for dt in sorted(set(b_dtypes) | set(h_dtypes)):
+                bc, hc = b_dtypes.get(dt, 0), h_dtypes.get(dt, 0)
+                if bc != hc:
+                    dtype_parts.append(f"{dt}: {bc} → {hc}")
+            if dtype_parts:
+                details_parts_init.append("dtype distribution: " + ", ".join(dtype_parts))
         changes.append(
             {
                 "type": "initializer_change",
-                "details": (f"initializer count {len(b_inits)} → {len(h_inits)}"),
+                "details": "; ".join(details_parts_init) or "initializers changed",
             }
         )
 

tests/_test_configs.py

@@ -326,6 +326,7 @@ def _base_config(config_cls=None, **overrides) -> ArchitectureConfig:
             "rope_local_base_freq": 10_000.0,
             # 2-layer test: 1 sliding + 1 full (must match TINY_LAYERS=2)
             "layer_types": ["sliding_attention", "full_attention"],
+            "sliding_window": 8,
             "global_head_dim": TINY_HEAD_DIM,
             "global_rope_theta": 10_000.0,
             "final_logit_softcapping": 30.0,
@@ -2055,6 +2056,7 @@ def _base_config(config_cls=None, **overrides) -> ArchitectureConfig:
             "attn_qk_norm": True,
             "rope_local_base_freq": 10_000.0,
             "layer_types": ["sliding_attention", "full_attention"],
+            "sliding_window": 8,
             "global_head_dim": TINY_HEAD_DIM,
             "global_rope_theta": 10_000.0,
             "global_partial_rotary_factor": 0.25,