feat(harness): scale-up fill check, config variants, multi-size dev gallery

Quality gates for adaptive layout:

- fill_metrics()/check_scale_up() in the safety harness: overflow catches
  content too big for a panel, but nothing caught content that stays tiny
  on panels >= 2x the plugin's declared design size. The check measures
  lit-content extents and warns (or fails, when a plugin opts into
  "fill_check": "strict" in test/harness.json) below 50% coverage on the
  doubled axis. Warn-only by default so no existing plugin breaks.

- harness.json "variants": extra runs with config overlays and their own
  golden dirs, so an opt-in mode (e.g. layout_mode: adaptive) is golden-
  tested beside the classic default. check_plugin.py loops base + variants
  and labels variant results mode@name.

- Dev preview server: GET /api/sizes (harness size sample), POST
  /api/render-matrix (render at up to 12 sizes in one call), size-preset
  dropdown, and an "All Sizes" side-by-side gallery in the preview UI.

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

src/plugin_system/testing/harness.py

@@ -73,6 +73,10 @@ class RenderResult:
     golden_ok: Optional[bool] = None
     golden_diff_pixels: int = 0
     golden_max_delta: int = 0
+    # fill / scale-up check (populated only for sizes >= 2x the design size)
+    fill_checked: bool = False
+    fill_ok: Optional[bool] = None       # False only in strict mode
+    fill_extent: Optional[Tuple[float, float]] = None  # (extent_x, extent_y)
 
     @property
     def size_label(self) -> str:
@@ -86,6 +90,8 @@ class RenderResult:
             return False
         if self.golden_checked and self.golden_ok is False:
             return False
+        if self.fill_ok is False:
+            return False
         return True
 
 
@@ -301,6 +307,74 @@ def compare_to_goldens(results: List[RenderResult], golden_dir: Path,
     return results
 
 
+# ---------------------------------------------------------------------------
+# Fill / scale-up check
+# ---------------------------------------------------------------------------
+#
+# Overflow catches content that is too BIG for a panel; nothing catches
+# content that stays tiny on a panel much larger than the plugin's design
+# size (e.g. 128x32 content in the corner of a 256x128 renders "green").
+# These helpers measure how much of the panel the lit content spans so the
+# harness can flag plugins that don't scale up.
+
+# A pixel counts as "lit" above this luminance — low enough to catch dim
+# content, high enough to ignore near-black noise.
+_LIT_THRESHOLD = 16
+# Content must span at least this fraction of an axis that is >= 2x the
+# design size. Lenient on purpose: margins are fine, a tiny corner is not.
+_MIN_FILL_EXTENT = 0.5
+
+
+def fill_metrics(image: Image.Image) -> Tuple[float, float, float]:
+    """Measure lit-content coverage: (extent_x, extent_y, ink_ratio).
+
+    extent_* are the lit bounding box's spans as fractions of the panel;
+    ink_ratio is the fraction of pixels lit (reporting only — sparse pixel
+    fonts legitimately have low ink ratios)."""
+    lit = image.convert("L").point(lambda p: 255 if p > _LIT_THRESHOLD else 0)
+    bbox = lit.getbbox()
+    if bbox is None:
+        return (0.0, 0.0, 0.0)
+    extent_x = (bbox[2] - bbox[0]) / image.width
+    extent_y = (bbox[3] - bbox[1]) / image.height
+    ink = sum(1 for p in lit.getdata() if p) / (image.width * image.height)
+    return (extent_x, extent_y, ink)
+
+
+def check_scale_up(results: List[RenderResult],
+                   design_size: Tuple[int, int] = (128, 32),
+                   min_extent: float = _MIN_FILL_EXTENT,
+                   strict: bool = False) -> List[RenderResult]:
+    """Flag renders that leave a big panel mostly empty.
+
+    For each result whose panel is at least 2x the design size on an axis,
+    require the lit content to span >= min_extent of that axis. Mutates the
+    results' fill_* fields. In the default warn-only mode fill_ok is left
+    None (reported, never failing); strict=True sets fill_ok=False, which
+    fails RenderResult.ok — opt in per plugin via harness.json
+    {"fill_check": "strict"} once its adaptive layout is in place.
+    """
+    design_w, design_h = design_size
+    for r in results:
+        if r.image is None or r.error is not None:
+            continue
+        check_x = r.width >= 2 * design_w
+        check_y = r.height >= 2 * design_h
+        if not (check_x or check_y):
+            continue
+        extent_x, extent_y, _ink = fill_metrics(r.image)
+        r.fill_checked = True
+        r.fill_extent = (round(extent_x, 3), round(extent_y, 3))
+        underfilled = ((check_x and extent_x < min_extent)
+                       or (check_y and extent_y < min_extent))
+        if underfilled and strict:
+            r.fill_ok = False
+        elif not underfilled:
+            r.fill_ok = True
+        # warn-only underfill: fill_ok stays None; fill_extent tells the story
+    return results
+
+
 def write_goldens(results: List[RenderResult], golden_dir: Path) -> int:
     """Write each successfully-rendered result to its golden path. Returns count."""
     written = 0

src/plugin_system/testing/loading.py

@@ -56,7 +56,15 @@ def load_harness_spec(plugin_dir: Union[str, Path]) -> Dict[str, Any]:
           "config":     {...},            # config overrides
           "mock_data":  "fixtures/mock.json",  # path (relative to plugin dir) to cache fixtures
           "freeze_time": "2025-08-01 15:25:00",
-          "skip_update": false
+          "skip_update": false,
+          "fill_check": "warn",           # or "strict": underfilled big panels FAIL
+          "variants": [                   # extra runs with config overlays and
+            {                             # their own golden dirs — e.g. an
+              "name": "adaptive",         # opt-in adaptive mode tested beside
+              "config": {"layout_mode": "adaptive"},   # the classic default
+              "golden_dir": "test/golden-adaptive"
+            }
+          ]
         }
     Returns {} when no harness.json exists.
     """