Read docs/PROMPT_scatter_hover.md

Prompt — hover tooltips on the fitness × synergy scatter (highlighted dots)

Add hover to the scatter so the highlighted dots (winner, p16, cell_cycle) show their genes + key scores. Frontend-only. Depends on PROMPT_fitness_synergy_scatter.md being in (it draws the canvas). Verified the data is all on the module row. No engine/API/airgap change. tsc after.

Scope — hover only the highlighted dots

Hit-test only the highlighted points (winner + cell_cycle + p16/immune groups) — that's a few dozen, so it's trivially fast, and it keeps the reveal bounded. Do NOT hover-test the ~6,000 faint background dots (they're not revealed, and revealing all of them would break the airgap).

Hit-testing on the canvas

• When drawing the highlighted dots, also record each one's screen position + its module: { cx, cy, module } in a ref array.
• On mousemove over the canvas, find the nearest highlighted point within ~8px of the cursor; if found, show the tooltip for that module; on mouseout / none-in-range, hide it.
• Tooltip = an absolutely-positioned HTML div over the chart container, following the cursor (clamp to stay on-screen).

Tooltip content

For the hovered group show:

• Genes (revealed symbols), e.g. MCM5, MCM2 (winner: C11orf85, ZFR2).
• Source tag: winner / p16 / cell_cycle (or immune / MMR for CRC) — colour-matched to the dot.
• GP fitness = m.gp_fitness
• Combined AUROC = m.combined_holdout
• Coherence = m.coherence
• Synergy = m.combined_holdout − max(per_gene single_gene_metric) (the same value the table/column uses; reuse the shared helper, don't recompute inconsistently).
• (Optionally size = m.size.) Format every number through the existing fmtFit so non-finite shows "—".

Revealing the gene symbols (bounded)

• The highlighted groups may include modules not on the current table page, so their symbols may not be revealed yet. On scatter mount, batch-reveal the union of the highlighted groups' gene_ids in one postReveal call (winner + all ref_sets-tagged groups). This is bounded (a few dozen groups × a few genes — ~100-200 ids), same discipline as revealing the reference sets. Cache symbolByOpaque and use it in the tooltip.
• Never reveal the background groups' genes.

CONSTRAINTS

• Frontend-only; scores are already on the row, symbols via a bounded reveal of the highlighted groups only. No engine/API/airgap change.
• Keep hover restricted to highlighted dots (fast + bounded reveal).

Checkpoint

• Hovering a winner / p16 / cell_cycle dot shows a tooltip with its gene symbols + GP fitness, Combined AUROC, Coherence, Synergy (and source).
• Background dots are not hover-targets; no full-map reveal.
• tsc clean; no API/airgap change.