feat(host_agent): lazy-load full-metrics charts + live-poll current state

The full-metrics page rendered once server-side with the history query inline,
so the charts blocked first paint and nothing refreshed without a reload (it
reads the latest snapshot the server holds — the agent pushes ~every 30s).

Split it into a shell + two HTMX fragments:
- Shell (host_detail.html): header + shared time-range toggle + two containers;
  paints instantly.
- Current state (/<id>/metrics → _host_metrics.html): identity + gauges +
  per-core + filesystems + interfaces/disks + temps, from the DISTINCT ON latest
  query. hx-trigger "load, every 15s" → live numbers at ~the agent cadence.
- History charts (/<id>/charts → _host_charts.html): the 3 charts + Chart.js,
  from the date_bin history query. hx-trigger "load, every 60s, rangeChange" so
  they lazy-load (never block paint), refresh slowly, and follow the range
  selector. Leak-safe: previous Chart instances are destroyed before re-render.
- Range toggle switched from full-reload links to the shared _time_range.html
  (setTimeRange + rangeChange), so only the fragments refetch.
- routes: host_detail (shell) + host_detail_metrics + host_detail_charts, with a
  _split_host_metrics helper.
- tests/test_templates_parse.py now also parses plugin templates (these
  fragments aren't rendered in the unit lane).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016Jg27rgypiW2efULXJDtMC

tests/test_templates_parse.py

@@ -11,12 +11,24 @@ import jinja2
 
 import steward
 
-_ROOT = pathlib.Path(steward.__file__).parent / "templates"
+_REPO = pathlib.Path(steward.__file__).parent.parent
+_CORE_TEMPLATES = _REPO / "steward" / "templates"
+_PLUGINS = _REPO / "plugins"
 
 
 def test_all_steward_templates_parse():
     env = jinja2.Environment()
-    files = sorted(_ROOT.rglob("*.html"))
+    files = sorted(_CORE_TEMPLATES.rglob("*.html"))
     assert files, "no steward templates found"
     for f in files:
         env.parse(f.read_text())  # raises TemplateSyntaxError on a bad tag
+
+
+def test_all_plugin_templates_parse():
+    # First-party plugins ship templates too (host_agent fragments, docker, …);
+    # they aren't rendered in the unit lane, so parse them here to catch a bad tag.
+    env = jinja2.Environment()
+    files = sorted(_PLUGINS.rglob("templates/**/*.html"))
+    assert files, "no plugin templates found"
+    for f in files:
+        env.parse(f.read_text())