perf: optimize CaDecon iteration hot paths

[daharoni]

Summary

Performance optimizations

• Batch SolidJS signals (batch() + bulkUpdateTraceResults): 400 reactive graph traversals per iteration → 1, the highest-impact change
• Shallow snapshot in snapshotIteration: eliminates ~3.2MB of deep-copied typed arrays per iteration (safe because TraceResultEntry objects are replaced, never mutated)
• Eliminate double copy in trace buildDispatch: transfer job.trace directly instead of allocating a copy (~1.6MB saved per iteration)
• Pre-allocated typed arrays in dispatchKernelJobs: two-pass approach replaces push(...spread) which risked stack overflow for >65k samples
• RasterOverview inner loop: hoist cell/rowBase outside inner px loop, inline dataIndex, remove unused import (~3M fewer Math.floor + function calls at 2x DPR)
• HistogramCard Math.min/max: manual loop prevents RangeError for arrays >65k elements

Draggable grid resizer

• New ResizableGrid component wrapping the 2x2 plot layout
• Drag handles at the column divider, row divider, and center intersection
• Center handle moves both axes; edge handles move one axis
• Double-click any handle to reset to default (1/3 + 2/3 columns, 50/50 rows)
• Clamped to 15–85% range; hidden on mobile (stacked layout)

Test plan

• npm run typecheck passes (only pre-existing errors in unrelated files)
• npm run build succeeds
• Manual: load app, import data, run full iteration loop
• Progress bar updates smoothly
• Trace inspector shows correct traces
• Iteration scrubber navigates history correctly (shallow snapshot change)
• Convergence/trend charts update each iteration
• Alpha/PVE distributions update (batching change)
• Raster overview renders correctly (inner loop change)
• No console errors or RangeErrors
• Drag grid dividers to resize panels; double-click to reset
• Center handle moves both axes simultaneously

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