issue #3137 : sample target hops and show data preview icons on hop arrows

[mattcasters]

What

Implements hop-level data sampling and preview icons for target hops (Filter Rows, Switch/Case, Java Filter, executors, etc.), addressing #3137.

Engine

• New IRowToListener / RowToAdapter with rowWrittenTo(IRowMeta, Object[], IRowSet) so destination-aware listeners can keep mixed layouts apart
• BaseTransform.handlePutRowTo() notifies row-to listeners (still does not call IRowListener, which mixed layouts on target hops)
• Wired through ITransform, IEngineComponent, EngineComponent
• Beam/Spark putRowTo aligned to IRowToListener instead of incorrectly firing IRowListener

GUI sampling

• PipelineRowSamplerHelper also attaches hop samplers keyed by origin/destination
• Hop sample buffers stored alongside transform samples (including drill-down sniffers)

Canvas

• Clickable data-grid icons on hops at ~30% from the source transform (AreaType.HOP_OUTPUT_DATA)
• Tooltip shows hop endpoints and row count; click opens the existing rows dialog for that hop

Why

Target-stream transforms use putRowTo(), which never called row listeners, so Filter/Switch/Case had no usable output preview. Sampling per hop (as discussed on the issue) makes data flow transparent on every target branch.

How to test

1. Run a local pipeline with GUI sampling enabled (First/Last/Random, not None)
2. Include Filter Rows or Switch/Case with target hops
3. After the run, data icons appear on those hops near the source
4. Hover for count; click to inspect sampled rows for that hop only

Related

Fixes #3137