perf(vegas): trace the content path at DEBUG instead of INFO

plugin_adapter narrates every step of acquiring content from every plugin --
"Has get_vegas_content", "Native: calling get_vegas_content()", "Native
content returned None", "Has scroll_helper", per-item sizes -- once per plugin
per cycle, all at INFO.

Measured on a live rig: 13,408 log lines an hour, of which 13,366 were INFO
and 35 were WARNING. Roughly 223 lines a minute of string formatting on a Pi
that is also driving the panel, written through journald to the SD card, with
the 35 lines that actually indicate a problem buried among them.

Top repeated messages in that hour:

    717  Scroll progress: elapsed=... total_scrolled=.../... px
    399  [plugin] --> INCLUDED in Vegas scroll
    323  [plugin] content_type=static, display_mode=fixed
    195  [plugin] Has get_vegas_content: True
    195  [plugin] Native: calling get_vegas_content()
    168  [plugin] Native: get_vegas_content() returned None
    168  [plugin] Native content returned None        <- the same fact, twice

54 logger.info calls in plugin_adapter become logger.debug, along with the
per-frame scroll-progress line in scroll_helper. Together those are 3,174 of
the 13,408 lines an hour, a 23% cut, and the ~3,600 odds-manager lines are
addressed separately by ledmatrix-plugins#300.

Nothing is lost: the 19 warning/error/exception calls in the module are
untouched, so real failures still surface at their own level. This is a
logging-level change only -- no control flow, no behaviour.

One INFO call is deliberate and stays. The padding-strip message picks its
level at runtime (`logger.warning if (left and right) else logger.info`) and
test_vegas_plugin_adapter.py pins that choice; it survives because it is not a
direct logger.info call site. That test still passes.

Mutation-checked both ways: reintroducing a single INFO trace fails the guard,
and demoting the warning/error calls along with the trace fails a second guard
written for exactly that mistake. 537 vegas and scroll tests pass.

test/test_vegas_log_volume.py

@@ -0,0 +1,63 @@
+"""The Vegas content path must trace at DEBUG, not INFO.
+
+plugin_adapter narrates every step of acquiring content from every plugin --
+"Has get_vegas_content", "Native: calling get_vegas_content()", "Native content
+returned None", "Has scroll_helper", the per-item sizes -- and it does that for
+each plugin on each cycle.
+
+Measured on a live rig: 13,408 log lines an hour, of which 13,366 were INFO and
+35 were WARNING. plugin_adapter alone produced 2,457 of them. That is ~223
+lines a minute of string formatting on a Pi that is also driving the panel, all
+of it written through journald to the SD card, and it buries the 35 lines that
+actually indicate a problem.
+
+Nothing is lost by moving it to DEBUG: the 19 warning/error/exception calls in
+the module are untouched, so real failures still surface at their own level.
+
+One INFO call is deliberate and stays -- the padding-strip message chooses its
+level at runtime (`logger.warning if (left and right) else logger.info`) and
+test_vegas_plugin_adapter.py pins it.
+"""
+import ast
+from pathlib import Path
+
+import pytest
+
+ADAPTER = (Path(__file__).resolve().parent.parent / "src" / "vegas_mode"
+           / "plugin_adapter.py")
+
+
+def _info_calls(path):
+    """Direct logger.info(...) call sites in a module."""
+    tree = ast.parse(path.read_text(encoding="utf-8"))
+    found = []
+    for node in ast.walk(tree):
+        if (isinstance(node, ast.Call)
+                and isinstance(node.func, ast.Attribute)
+                and node.func.attr == "info"
+                and getattr(node.func.value, "id", None) == "logger"):
+            found.append(node.lineno)
+    return found
+
+
+def test_the_content_path_does_not_trace_at_info():
+    calls = _info_calls(ADAPTER)
+    assert not calls, (
+        "plugin_adapter should trace at DEBUG; found logger.info at lines "
+        f"{calls}. This path runs per plugin per cycle and its output goes to "
+        "the SD card via journald."
+    )
+
+
+def test_real_failures_still_have_a_level_of_their_own():
+    """Demoting the trace must not have swept up the error reporting."""
+    source = ADAPTER.read_text(encoding="utf-8")
+    loud = sum(source.count(f"logger.{level}(")
+               for level in ("warning", "error", "exception"))
+    assert loud >= 15, f"only {loud} warning/error/exception calls remain"
+
+
+def test_the_deliberate_runtime_chosen_level_survives():
+    """The padding-strip message picks its level at runtime; leave it alone."""
+    source = ADAPTER.read_text(encoding="utf-8")
+    assert "logger.warning if (left and right) else logger.info" in source