[FEATURE] Frontend: render EAT predicted-by-transfer annotations (value-level overlay)

[tsenoner]

Summary

Render the per-cell Embedding Annotation Transfer (EAT) predictions that the backend now writes into .parquetbundle files as a new value-level "predicted-by-transfer" layer in the web app — distinct from, and orthogonal to, PR #272's column-level ⚡ "predicted-by-model" badge.

This is the frontend follow-up to the backend work in tsenoner/protspace#55 (concept: tsenoner/protspace#54).

Background — the two orthogonal axes

• Axis A (existing, feat(annotations): mark predictions and surface per-annotation docs #272): column-level — "this whole annotation column is a model output" (Biocentral/Phobius/TED). Keep the ⚡ badge / source grouping / info-popover unchanged.
• Axis B (new, this issue): cell-level — "this specific protein's value in column COL was transferred from a neighbour, with confidence X and source Y."

Never overload the ⚡ badge to mean both.

What the backend now emits

For each transferred column COL, protspace transfer appends three columns to the bundle's annotations table (the curated COL is left untouched; protein_id id column preserved → backward compatible):

Column	Type	Meaning
COL__pred_value	string	the transferred label
COL__pred_confidence	float32	reliability index in [0, 1]
COL__pred_source	string	nearest reference protein id

A cell is predicted for COL when COL is empty/blank but COL__pred_value is non-null.

UX (from design spec §9)

1. Scatter — primary cue is shape, not colour: predicted cells render as hollow (outline-only) markers in the same category hue; observed cells stay filled. Confidence → opacity ramp (alpha = lerp(0.25, 0.9, confidence)), optionally size; desaturate toward grey below ~0.3. Never colour-only (accessibility).
2. Tooltip — provenance line, e.g. ⚡ Predicted: Neurotoxin (82%) — transferred from P12345 via ProtT5, k=1, with an inline confidence bar and the source id as a click-to-select target.
3. Legend — a separate "Predicted (transferred)" sub-section (filled "Observed" / hollow "Predicted by EAT" swatches + live counts). Do not merge into the ⚡ header badge.
4. Global control — "Show predicted annotations" toggle + confidence-threshold slider (High >80 / Med 50–80 / Low <50); below-threshold cells fade rather than vanish; persist in legend settings.

Concrete hooks

• packages/utils/src/types.ts — add optional annotation_predicted (parallel to annotation_scores / annotation_evidence).
• packages/core/src/components/data-loader/utils/bundle.ts — detect *__pred_* columns, pivot into the data model (old bundles lack them → no overlay).
• packages/core/src/components/scatter-plot/style-getters.ts — getOpacity / getPointShape branch on predicted cell + confidence.
• packages/core/src/components/scatter-plot/webgl-renderer.ts — a_predicted attribute + hollow/ring render (reuse the edge-distance math, strokeWidth = 0.15).
• packages/core/src/components/scatter-plot/protein-tooltip.ts — provenance line + clickable source.
• packages/core/src/components/legend/legend-renderer.ts — "Predicted (transferred)" sub-section.

Process

• Should be its own OpenSpec change building on feat(annotations): mark predictions and surface per-annotation docs #272's annotation-metadata / annotation-presentation capabilities.
• Caveat to surface in UI/docs: the reliability index is ProtT5-calibrated; it is monotone-but-uncalibrated for other embedders — present as a ranking, not a probability.

Acceptance criteria

• Bundle parser surfaces predicted value / confidence / source per cell.
• Scatter shows hollow markers + confidence opacity for predicted cells; observed cells unchanged.
• Tooltip shows provenance with a clickable source id.
• Legend "Predicted (transferred)" sub-section with counts.
• Global toggle + confidence-threshold slider; persisted in settings.
• Grayscale/PNG export keeps the hollow-vs-filled distinction.
• Accessibility: non-colour encoding; keyboard-operable slider.

References

• Design spec (§9): docs/superpowers/specs/2026-06-11-eat-annotation-transfer-design.md in tsenoner/protspace
• Backend PR: feat: protlabel EAT engine + protspace transfer subcommand protspace#55 · Concept: [FEATURE] EAT — Embedding Annotation Transfer (protlabel lookup table) protspace#54 · Builds on: feat(annotations): mark predictions and surface per-annotation docs #272