Add cross-size/cross-screen plugin safety harness (#361)

* feat(testing): add cross-size/cross-screen plugin safety harness

Render every plugin across all supported matrix sizes (64x32, 128x32,
128x64, 256x32) and every declared screen, failing on crashes, content
drawn past the panel edge, or visual drift vs committed golden images.

- BoundsCheckingDisplayManager: oversized-canvas overflow detection
- harness.py: multi-size/multi-screen render engine + golden compare
- scripts/check_plugin.py: CLI (functional+bounds, --out-dir, --update-golden,
  --freeze-time); render_plugin.py refactored onto shared loading helpers
- test/plugins/test_harness.py + test_plugin_matrix.py (parametrized,
  honors per-plugin test/harness.json; skips when no plugins present)
- MockCacheManager.cache_dir so cache-dir-using plugins load headlessly
- .github/workflows/test.yml + docs/plugin-safety-harness.md

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* fix(testing): address PR review feedback on plugin safety harness

- check_plugin: friendly error for non-numeric --sizes; reject non-object
  --config / --mock-data JSON; sanitize plugin mode before using as a
  filename; stop --update-golden from masking crash/overflow failures
- bounds_display_manager: pad the canvas out to the largest supported panel
  (not a fixed 16px) so far-overshoot coordinates are caught, not clipped
- harness: merge config_schema defaults inside render_plugin_matrix; surface
  update() failures as a non-fatal warning + result field instead of a debug
  log; sanitize mode in golden_path
- loading: fail fast when harness.json references a missing mock_data fixture
- mocks: clean up the per-instance temp cache dir via weakref.finalize
- test_plugin_matrix: add a discovery guard that fails when
  LEDMATRIX_REQUIRE_PLUGINS=1 but none found (still skips locally); type hints
- bound test deps with upper version pins for deterministic CI

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* feat(testing): render plugins across arbitrary panel sizes, not a fixed list

Addresses maintainer feedback that there is no canonical set of supported
panel sizes — a build can be any size/configuration (square, 2x2, 4x4, 8x2,
long strips, tall stacks).

- sizes.py: SUPPORTED_SIZES -> DEFAULT_TEST_SIZES (back-compat alias kept),
  reframed as a representative SAMPLE of real panel-grid arrangements rather
  than an authoritative list; add parse_size_token / coerce_sizes /
  resolve_test_sizes helpers
- sizes are now fully overridable: LEDMATRIX_TEST_SIZES env (global, e.g. test
  on your exact hardware) > per-plugin harness.json "sizes" > default sample;
  CLI --sizes unchanged
- bounds_display_manager: pad the canvas to the largest panel IN THE CURRENT
  RUN (via overflow_extent) instead of a hardcoded max, so cross-size overflow
  detection scales to whatever sizes a run uses
- harness: compute per-run extent and thread it into the bounds manager
- tests: arbitrary-shape + size-parsing/precedence coverage
- docs: rewrite "Supported sizes" -> "Sizes: a sample, not a fixed list"

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* fix(testing): fail the harness on non-connectivity update() errors

Addresses the remaining review thread: recording every update() exception as a
non-fatal warning still let a real update() regression pass green as long as
display() survived. Now update() failures are classified — a tolerated set of
connectivity errors (ConnectionError/TimeoutError/socket/ssl/urllib/http/
requests) is recorded non-fatally (expected with no network in CI), while any
other exception is treated as a genuine bug and fails that render.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* ci(security): pin actions to SHAs and disable checkout credential persistence

Addresses the CodeRabbit/zizmor workflow-hardening finding: pin
actions/checkout and actions/setup-python to full commit SHAs and set
persist-credentials: false on checkout to reduce supply-chain and
token-exposure risk.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* fix(testing): validate positive sizes; narrow requests import except

Two review findings:
- sizes.py: parse_size_token / coerce_sizes now reject non-positive
  dimensions (0x32, -64x32) with a clear message instead of passing invalid
  sizes downstream (CodeRabbit).
- harness.py: the optional `requests` import now catches ImportError
  specifically and logs instead of `except Exception: pass`, clearing the
  Codacy medium "Try, Except, Pass" (harness.py L52) and Ruff S110/BLE001.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/plugin_system/testing/loading.py

@@ -0,0 +1,82 @@
+"""
+Shared helpers for loading a plugin headlessly.
+
+Used by scripts/render_plugin.py, scripts/check_plugin.py, and the harness so
+plugin discovery / manifest / config-default logic lives in exactly one place.
+"""
+
+import json
+from pathlib import Path
+from typing import Any, Dict, Optional, Sequence, Union
+
+
+def find_plugin_dir(plugin_id: str, search_dirs: Sequence[Union[str, Path]]) -> Optional[Path]:
+    """Find a plugin directory by searching multiple paths."""
+    from src.plugin_system.plugin_loader import PluginLoader
+    loader = PluginLoader()
+    for search_dir in search_dirs:
+        search_path = Path(search_dir)
+        if not search_path.exists():
+            continue
+        result = loader.find_plugin_directory(plugin_id, search_path)
+        if result:
+            return Path(result)
+    return None
+
+
+def load_manifest(plugin_dir: Union[str, Path]) -> Dict[str, Any]:
+    """Load and return manifest.json from a plugin directory."""
+    manifest_path = Path(plugin_dir) / 'manifest.json'
+    if not manifest_path.exists():
+        raise FileNotFoundError(f"No manifest.json in {plugin_dir}")
+    with open(manifest_path, 'r') as f:
+        return json.load(f)
+
+
+def load_config_defaults(plugin_dir: Union[str, Path]) -> Dict[str, Any]:
+    """Extract default values from a plugin's config_schema.json (empty if none)."""
+    schema_path = Path(plugin_dir) / 'config_schema.json'
+    if not schema_path.exists():
+        return {}
+    with open(schema_path, 'r') as f:
+        schema = json.load(f)
+    defaults: Dict[str, Any] = {}
+    for key, prop in schema.get('properties', {}).items():
+        if isinstance(prop, dict) and 'default' in prop:
+            defaults[key] = prop['default']
+    return defaults
+
+
+def load_harness_spec(plugin_dir: Union[str, Path]) -> Dict[str, Any]:
+    """Optional per-plugin harness settings from <plugin>/test/harness.json.
+
+    Lets a plugin opt into golden-image testing by declaring how to render it
+    deterministically. All keys optional:
+        {
+          "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
+        }
+    Returns {} when no harness.json exists.
+    """
+    spec_path = Path(plugin_dir) / 'test' / 'harness.json'
+    if not spec_path.exists():
+        return {}
+    with open(spec_path, 'r') as f:
+        spec = json.load(f)
+
+    # Resolve mock_data path and inline its contents for convenience.
+    mock_rel = spec.get('mock_data')
+    if mock_rel:
+        mock_path = Path(plugin_dir) / mock_rel
+        if not mock_path.exists():
+            # A declared-but-missing fixture is a harness config error: failing
+            # loudly beats silently rendering the plugin with no mock data.
+            raise FileNotFoundError(
+                f"harness.json references mock_data '{mock_rel}' but "
+                f"{mock_path} does not exist"
+            )
+        with open(mock_path, 'r') as mf:
+            spec['mock_data_contents'] = json.load(mf)
+    return spec