CommOverTime: Add average message size line views

[lvkale]

Adds average-message-size views to Communication Over Time, plus the line-graph infrastructure they need.

Communication Over Time gets a second radio row — Avg Size Sent / Recv / External Recv / External Node Recv — plotting per-interval bytes-per-message as one unstacked line per entry method (averages are not additive, so stacked bars would be wrong):

• Y values are log2(bytes) since message sizes span orders of magnitude; ticks are labeled with actual sizes (2, 4, ... 512, 1K, 2K, ... 1M).
• Only the top 10 EPs by total byte volume are plotted; EPs whose messages all fall in a single interval are omitted as outliers. The status label reports what was omitted.
• Intervals with no messages break the line rather than plotting zero; rare messages appear as dots.
• A "Show Legend" checkbox (auto-enabled entering an avg view) opens the movable Legend window listing exactly the plotted EPs with overall average size and message count. Bar views get a top-10-by-total legend.
• Avg mode forces an unstacked line chart, a white background, and horizontal gridlines, restoring previous settings on leaving. Rate scalings are disabled (bytes/message is already a ratio).

Graph infrastructure (reusable by other tools):

• NaN values in line data are gaps: lines break instead of dropping to zero, and isolated samples are marked with a dot.
• Hover popups now work on unstacked line graphs (nearest-series hit test).
• Optional light horizontal gridlines at every other y tick, drawn under the data.
• GraphPanel.selectGraphType() for programmatic graph-type switching with control sync; GenericGraphWindow gains getGraphPanel() and a setYAxis(YAxis) overload for custom-labeled axes.

Tested interactively on a 480-PE trace across three review rounds.

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