Epoch reviewer Phase 0: ship epoch arrays to a static Canvas renderer

[jdpigeon]

What & why

Phase 0 of the Interactive Epoch Review UI (docs/epoch-review-ui-plan.md), forged from the plan locked via /plan-eng-review (§0). This half is read-only: it ships epoch data across the worker boundary and draws it, proving the data path and the Canvas 2D rendering choice. Click-to-reject, apply, and live ERP are Phase 1 (next PR).

Locked architecture decisions this implements: OQ1 = Canvas 2D + DOM/SVG overlay, OQ2 = extend the dataKey pattern (no runPython RPC).

Data path (all new)

CleanComponent "Load Dataset"
  → loadEpochsEpic → requestEpochArrays('raw_epochs')
  → worker: get_epochs_arrays() writes a float32 buffer to MEMFS + returns metadata
  → worker posts { buffer, results, dataKey:'epochArrays' }  ← zero-copy transferable
  → pyodideMessageEpic → SetEpochArrays → reducer
  → CleanContainer (...state.pyodide) → CleanComponent → EpochReviewer draws

Changes

• utils.py — get_epochs_arrays(epochs, out_path) → float32 C-order [epoch][channel][time] buffer + metadata (ch_names EEG-only with the Marker/stim channel excluded via pick_types(eeg=True), sfreq, times, event_codes, drop_log). plot_topo gains a palette param; the condition-color fallback is DRYed to one _DEFAULT_CONDITION_PALETTE.
• webworker.js — the readFileAfter read-back is unified to post { buffer, results }; the PR Fix broken Clean→Analyze round-trip: persist cleaned epochs to host disk #222 savedEpochs write-back migrated to the same field (verified still working).
• conditionPalette.ts (new) — canonical condition palette, single source for both the Python matplotlib plots (JSON-injected into plot_topo/plot_conditions) and the React reviewer.
• redux — SetEpochArrays action + EpochArraysMeta type + reducer state (reset on experiment cleanup).
• EpochReviewer.tsx (new) — Canvas 2D traces (epochs side-by-side, channels stacked) + DOM overlay (channel/epoch labels, dividers), viewport windowing (first 8 epochs), spike-preserving min/max downsampling, condition coloring, empty state, devicePixelRatio scaling. Pure decode/downsample helpers in epochArrays.ts.

Out of scope (Phase 1)

Click-to-reject, apply_rejection, live ERP preview, scroll/scale navigation.

Verification

• npm run typecheck — 0 errors
• eslint (changed files) — 0 errors (1 pre-existing warning)
• npx vitest run — 37/37 pass (incl. 9 new decode/downsample/color-index cases)
• native pytest tests/analysis/ — 10/10 pass (4 new: buffer decodes to get_data(), Marker excluded, byte length, event codes; 6 existing, no regression)
• npm run build (electron-vite) — green
• Independent adversarial review traced the full data path, buffer-layout match, Marker exclusion (no off-by-one), transferable safety, palette single-source, and confirmed no Phase-1 scope drift. Verdict: PASS.

Stacked on epoch-cleaned-epochs-writeback (PR #222). 🤖 Generated with Claude Code