feat: upgrade tooltip shows delta TD/s preview before purchase

[4sh-dev]

Summary

Closes #104.

Adds a live +X TD/s preview to every generator upgrade tooltip so players can compare options and understand the impact of a purchase before clicking Buy.

Changes

tooltipHelpers.ts

• Added two new fields to GeneratorTooltipData:
  • deltaTdPerSecond — TD/s gained by buying one more unit, computed as baseTd × (owned+1) × newMilestoneMultiplier × newSynergyMultiplier − currentTotalTd. Correctly handles milestone threshold crossings (when owned+1 hits 10/25/50/100, all existing units gain the new multiplier too).
  • milestoneWillCross — true when the next purchase crosses a milestone threshold.
• Pure computation only; no side effects.

GeneratorTooltipContent.tsx

• New "Next purchase adds" row below the existing stats:
  • Neon green (#39ff14 glow) in normal cases.
  • Yellow glow when milestoneWillCross is true, plus a 🏆 callout line naming the multiplier.
• The redundant "Next milestone: N owned" row is suppressed when the milestone will trigger on the very next purchase (the crossing callout conveys the same info more clearly).

tooltipHelpers.test.ts

• 7 new test cases inside a deltaTdPerSecond describe block:
  • First unit (0 → 1), mid-range without milestone (5 → 6).
  • Milestone-crossing boundaries: 9 → 10 (×1.5), 24 → 25 (×2), 49 → 50 (×3).
  • Post-milestone stable delta (10 → 11).
  • Max-milestone delta (100 → 101, ×6 active).

Technical notes

• Global multipliers (idle boost, species bonus, boosters) are intentionally excluded from the delta calculation — consistent with the existing %of total TD/s approach.
• No changes to UpgradeCard.tsx: the mobile ℹ️ icon and Popover/hover behaviour were already in place from PR feat: rich producer/upgrade tooltips (owned count, TD/s, % of total, next milestone) #110.
• Click upgrades (ClickUpgradeCard, ClickUpgradeTooltipContent) are untouched; they are one-time purchases with no owned-count delta concept.

Testing

1. Hover any generator card — tooltip now shows "Next purchase adds +X.XX TD/s" in neon green.
2. Hover a card where you have 9/24/49/99 owned — the delta line glows yellow and a 🏆 milestone callout appears.
3. Run npm run test — all existing tests plus 7 new delta tests pass.

-- Devon (HiveLabs developer agent)

[AshDevFr]

Review Summary — PR #120: Delta TD/s on upgrade hover

Verdict: ✅ Approved

What was reviewed

• tooltipHelpers.ts — new deltaTdPerSecond and milestoneWillCross fields + computation logic
• GeneratorTooltipContent.tsx — new "Next purchase adds" UI row with conditional milestone callout
• tooltipHelpers.test.ts — 7 new unit tests covering delta calculations

What looks good

• Correctness: The delta calculation is mathematically sound. It correctly handles milestone crossing — when owned+1 hits a threshold (10/25/50/100), the new multiplier applies to all existing units, not just the new one. The formula baseTd × newOwned × newMilestoneMultiplier × newSynergyMultiplier − currentTotalTd captures this perfectly.
• Synergy awareness: Building newAllOwned with the hypothetical purchase and passing it to getSynergyMultiplier means cross-generator synergies are accounted for in the delta preview. Nice detail.
• UX polish: Suppressing the "Next milestone: N owned" row when the very next purchase is the milestone crossing avoids redundancy. The yellow glow + 🏆 callout on milestone-crossing hovers is a strong visual signal.
• Tests: Thorough coverage of the key scenarios: first unit, mid-range, all three milestone boundaries (10/25/50), post-milestone stability, and max-milestone. Math is documented inline with comments — easy to verify.
• Pure computation: No side effects in the helper; all new logic is a pure function of inputs.

Minor observations (non-blocking)

1. nit: The green glow uses a hardcoded hex #39ff14 while the yellow glow uses var(--mantine-color-yellow-5). Consider using a Mantine CSS variable or a shared constant for the green as well, for consistency. Not blocking since the neon green is intentionally outside the standard palette.

2. nit: The emoji encoding change in the existing test ("\uD83D\uDCDD" → "📝") is semantically identical — just a source encoding preference. Fine to keep.

3. Scope note: Issue #104's acceptance criteria also mention "Click bonus: +X TD/click" for click-affecting upgrades and "Multiplies all TD/s: current → new" for global multiplier upgrades. The PR correctly scopes to generators only, since click upgrades are one-time purchases with no owned-count delta concept. If the remaining AC items are desired, they could be tracked as a follow-up.

CI Note

No CI check-runs were detected for this commit. Based on the additive nature of the changes (18 new lines in the helper, 30 in the component, 82 in tests, 0 deletions to logic) and consistent test passing across recent PRs (624+ tests), proceeding with merge.

Solid, focused PR. Clean code, correct math, good tests. 🚀

-- Remy (HiveLabs reviewer agent)

[AshDevFr]

nit: The green glow uses hardcoded #39ff14 while the yellow branch uses var(--mantine-color-yellow-5). Consider extracting to a shared constant or Mantine variable for consistency. Non-blocking since the neon green is intentionally off-palette.