chore: mark skins unsupported, fix stale docs and preview size, prepare 3.4.0

Skins: no current scoreboard plugin builds on src.base_classes, so the only
skin hook (SportsCore._render_game) never runs. The plugin schema endpoint no
longer injects the Visual Skin dropdown, the store hides and refuses
"type": "skin" registry entries, and GET /api/v3/skins reports
supported: false with a message. Stored skin config still loads and saves.
src/skin_system/ and its tests are unchanged apart from the support flag.

Docs: check_plugin.py/render_plugin.py examples use --plugin; document
BasePlugin.get_update_interval() and its interaction with the manifest
update_interval; CLAUDE.md drops the stale template line number and
recommends display_manager.width/height.

Preview size: new src/display_geometry.py holds the size computation and
defaults DisplayManager uses (double-sided applied, chain_length default 2).
The web preview, /display/current, Starlark magnify default, sync handshake
and two dev scripts use it.

Release: __version__ 3.4.0, CHANGELOG 3.4.0 section plus a 3.3.0 tag note.

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

test/test_display_geometry.py

@@ -0,0 +1,117 @@
+"""The web preview must be the size DisplayManager actually renders at.
+
+Before src/display_geometry.py, the preview endpoints computed
+``cols * chain_length`` by ``rows * parallel`` themselves, ignored
+``display.double_sided`` (so a double-sided panel previewed two screens side
+by side), and the ``chain_length`` fallback was 2 in DisplayManager but 1 in
+the Starlark magnify default and the sync handshake.
+"""
+
+import json
+from pathlib import Path
+from unittest.mock import MagicMock
+
+import pytest
+from flask import Flask
+
+from src.display_geometry import (
+    DEFAULT_CHAIN_LENGTH, DEFAULT_COLS, DEFAULT_PARALLEL, DEFAULT_ROWS,
+    logical_size, physical_size,
+)
+
+REPO_ROOT = Path(__file__).resolve().parents[1]
+
+
+def _config(hardware=None, double_sided=None):
+    display = {'hardware': hardware or {}}
+    if double_sided is not None:
+        display['double_sided'] = double_sided
+    return {'display': display}
+
+
+def test_defaults_match_the_config_template():
+    template = json.loads(
+        (REPO_ROOT / 'config' / 'config.template.json').read_text(encoding='utf-8'))
+    hw = template['display']['hardware']
+    assert (DEFAULT_ROWS, DEFAULT_COLS, DEFAULT_CHAIN_LENGTH, DEFAULT_PARALLEL) == (
+        hw['rows'], hw['cols'], hw['chain_length'], hw['parallel'])
+
+
+def test_missing_hardware_uses_the_template_defaults():
+    assert physical_size({}) == (128, 32)
+    assert logical_size({}) == (128, 32)
+
+
+def test_parallel_multiplies_height():
+    cfg = _config({'rows': 32, 'cols': 64, 'chain_length': 2, 'parallel': 2})
+    assert physical_size(cfg) == (128, 64)
+
+
+def test_double_sided_horizontal_previews_one_screen():
+    cfg = _config({'rows': 32, 'cols': 64, 'chain_length': 4, 'parallel': 1},
+                  {'enabled': True, 'copies': 2, 'axis': 'horizontal'})
+    assert physical_size(cfg) == (256, 32)
+    assert logical_size(cfg) == (128, 32)
+
+
+def test_double_sided_vertical_splits_parallel():
+    cfg = _config({'rows': 32, 'cols': 64, 'chain_length': 2, 'parallel': 2},
+                  {'enabled': True, 'copies': 2, 'axis': 'vertical'})
+    assert logical_size(cfg) == (128, 32)
+
+
+def test_double_sided_that_does_not_divide_falls_back_to_physical():
+    cfg = _config({'rows': 32, 'cols': 64, 'chain_length': 2, 'parallel': 1},
+                  {'enabled': True, 'copies': 3, 'axis': 'horizontal'})
+    assert logical_size(cfg) == (128, 32)
+
+
+def test_disabled_double_sided_is_ignored():
+    cfg = _config({'chain_length': 4}, {'enabled': False, 'copies': 2})
+    assert logical_size(cfg) == (256, 32)
+
+
+@pytest.fixture
+def display_client(monkeypatch):
+    from web_interface.blueprints.api_v3 import api_v3
+    config_manager = MagicMock()
+    monkeypatch.setattr(api_v3, 'config_manager', config_manager, raising=False)
+    app = Flask(__name__)
+    app.config['TESTING'] = True
+    app.register_blueprint(api_v3, url_prefix='/api/v3')
+    with app.test_client() as client:
+        yield client, config_manager
+
+
+def test_display_current_reports_the_logical_size(display_client):
+    client, config_manager = display_client
+    config_manager.load_config.return_value = _config(
+        {'rows': 32, 'cols': 64, 'chain_length': 4, 'parallel': 2},
+        {'enabled': True, 'copies': 2, 'axis': 'horizontal'})
+
+    data = client.get('/api/v3/display/current').get_json()['data']
+
+    assert (data['width'], data['height']) == (128, 64)
+
+
+def test_display_current_defaults_chain_length_like_display_manager(display_client):
+    client, config_manager = display_client
+    config_manager.load_config.return_value = _config({'rows': 32, 'cols': 64})
+
+    data = client.get('/api/v3/display/current').get_json()['data']
+
+    assert (data['width'], data['height']) == (64 * DEFAULT_CHAIN_LENGTH, 32)
+
+
+def test_preview_callers_do_not_rederive_the_size():
+    """Every preview/size caller goes through display_geometry, so none of
+    them can drift back to a private chain_length default."""
+    for rel in ('web_interface/app.py',
+                'web_interface/blueprints/api_v3/display.py',
+                'web_interface/blueprints/api_v3/__init__.py',
+                'src/common/sync_manager.py',
+                'src/display_manager.py'):
+        source = (REPO_ROOT / rel).read_text(encoding='utf-8')
+        assert "get('chain_length', 1)" not in source, rel
+        assert 'get("chain_length", 1)' not in source, rel
+        assert 'cols * chain_length' not in source, rel