Add overflow handling: keep ordered content whole instead of rotating a window

The width budget split any oversized plugin by advancing a window each cycle.
That is right for interchangeable items — news headlines, odds, stock prices —
but wrong for ordered content: a league table showed ranks 1-6, then resumed at
7 two rotations later, which reads as out of order and out of context. Nobody
needs rank 23 in a ticker; they need the top of the table, every time.

overflow_mode chooses between them:

  rotate   — advance a window each cycle so everything is seen eventually
             (unchanged default)
  truncate — always show the start and drop the rest, keeping ordered content
             coherent. Records no window state, so every pass starts at the top.

Per-plugin vegas_overflow overrides the global setting, since one install has
both kinds of plugin. Also adds per-plugin vegas_max_width_screens, so content
that must stay whole can be given more room — or uncapped with 0 — without
lifting the cap on every ticker.

Applied on the test rig: f1-scoreboard and ledmatrix-leaderboard set to
truncate, and baseball given 4.5 screens because it was showing 8 of 9 games
when the whole slate needed only a little more room. Verified: F1 now reports
"the first 10 of 116 ... the rest are not shown", baseball has dropped out of
the budget log entirely, and stocks, odds-ticker and stock-news still rotate.

Also corrects the crop log, which claimed "window advances next cycle"
unconditionally and so misreported truncated crops. A test now pins the
behaviour behind the message: truncate must leave no offset recorded.

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

web_interface/blueprints/api_v3.py

@@ -926,7 +926,7 @@ def save_main_config():
                        'vegas_intra_plugin_gap', 'vegas_render_width_pct',
                        'vegas_min_content_separation', 'vegas_min_cut_gap',
                        'vegas_continuous_scroll', 'vegas_extend_threshold_screens',
-                       'vegas_smooth_scroll']
+                       'vegas_smooth_scroll', 'vegas_overflow_mode']
 
         if any(k in data for k in vegas_fields):
             if 'display' not in current_config:
@@ -951,6 +951,16 @@ def save_main_config():
 
             # max_plugin_width_ratio is the one fractional setting, so it is
             # handled outside the integer loop below.
+            if data.get('vegas_overflow_mode') not in ('', None):
+                mode = str(data['vegas_overflow_mode']).strip().lower()
+                if mode not in ('rotate', 'truncate'):
+                    return jsonify({
+                        'status': 'error',
+                        'message': "Invalid value for vegas_overflow_mode: "
+                                   "must be 'rotate' or 'truncate'"
+                    }), 400
+                vegas_config['overflow_mode'] = mode
+
             if data.get('vegas_extend_threshold_screens') not in ('', None):
                 try:
                     screens = float(data['vegas_extend_threshold_screens'])