ui: fix ts/dur deep-link selection for GPU slices

[suo]

Deep links carrying ts/dur query params (dev.perfetto.DeeplinkQuerystring) zoom to the right viewport but silently fail to select the target slice on GPU traces. Two independent causes:

1. UnionDataset.query() merges datasets whose joins differ.
Datasets are grouped for merging by source table and join alias only (getJoinSignature). The UI tracks created for grouped/split trace-processor tracks (e.g. GPU hardware queues whose overlapping slices TrackCompressor splits into parallel tracks) each join their own experimental_slice_layout depth table under the shared depth alias. The merge keeps only one of those join definitions (Object.assign), so the merged branch inner-joins every row through a single depth table and silently drops all rows that only exist in the other groups. resolveSqlEvents() then finds no track for the slice and selectSqlEvent() drops the selection without feedback. Any trace with two or more split tracks is affected.

Fix: include the join definition (from clause + unique flag) in the signature so only datasets with identical joins are merged. Refining the key can only prevent merges, so identically-joined datasets keep merging exactly as before; only the previously-corrupted case changes (each dataset becomes its own UNION branch, which is the definitional semantics).

2. dev.perfetto.GpuByProcess tracks are invisible to SQL-event resolution.
The plugin registers its per-process leaf tracks without rootTableName, so even a successfully found slice cannot resolve to them. Fix: pass rootTableName: 'slice', matching the id namespace the track's details panel (ThreadSliceDetailsPanel) already assumes for these datasets (gpu_slice ids are slice ids).

Testing

• New unit test (union dataset does not merge datasets whose joins differ) fails without the dataset.ts fix and passes with it; the other union-merge tests pass in both states (no behavior change for legitimate merges).
• ui/run-unittests: 148 files, 2537 passed / 1 skipped.
• ui/prettier --check and ui/eslint clean on touched files.
• End-to-end: on a real GPU trace (GpuRenderStageEvent kernels, one hardware queue split into 4 tracks by TrackCompressor), a deep link with visStart/visEnd/ts/dur previously zoomed correctly but selected nothing; with this change the slice is selected, the track auto-reveals, and the details panel shows the target kernel.

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