Align Vegas API bounds with validate(), fix audit config plumbing

Both from review feedback on #423.

The web API's accepted ranges disagreed with VegasModeConfig.validate(),
which is what actually gates Vegas starting:

  scroll_speed      1-100  -> 1-200   (a slider value of 150 returned 400)
  separator_width   0-500  -> 0-128
  target_fps        1-200  -> 30-200
  buffer_ahead      1-20   -> 1-5

The three loose ones were the dangerous direction: the value saved with a
200, then VegasModeCoordinator.start() failed validation with only a log
line, so the ticker silently never ran. The UI already matched validate() in
all four cases, so the API was the odd one out.

test_vegas_api_bounds_match_validate parses the numeric_fields map out of
api_v3 and asserts every bound against validate(), plus that validate()
accepts both endpoints and rejects just outside them, so these cannot drift
apart again. That test immediately caught a missing upper bound on
min_plugin_width, now added — unbounded it would drop every segment and
leave a blank ticker.

Separately, vegas_audit.py constructed PluginAdapter without the config, so
it fell back to VegasModeConfig() defaults and would report trimming and
width-budget behaviour that differed from the user's config.json. It now
passes the loaded config exactly as the coordinator does. This is the same
class of drift the explicit lead_gap and grouping arguments already guard
against. Output is unchanged on a rig whose config matches the defaults.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KEZK1P1Q1fu5pcuVrkrCFZ

src/vegas_mode/config.py

@@ -230,6 +230,11 @@ class VegasModeConfig:
         if self.min_plugin_width < 0:
             errors.append(
                 f"min_plugin_width must be >= 0, got {self.min_plugin_width}")
+        # Bounded because every segment narrower than this is dropped — an
+        # unbounded value would discard every plugin and leave a blank ticker.
+        if self.min_plugin_width > 512:
+            errors.append(
+                f"min_plugin_width must be <= 512, got {self.min_plugin_width}")
 
         if self.lead_in_width < 0:
             errors.append(