feat(dex): add fill probability estimator for limit orders

[elgifts12]

Estimate the likelihood a proposed limit order fills using two signals from current order book depth and recent trade cadence:

• Depth score: walks book levels from best price toward the limit; aggressive prices (inside spread) score ~1, deep passive limits ~0. Formula: exp(-3 * depthFraction)
• Cadence score: trades-per-minute at-or-better-than limit price in a 5-min window. Formula: 1 - exp(-0.5 * tpm)

Both combine via weighted geometric mean (depth 60%, cadence 40%) into a [0,1] probability with tier labels (very_high/high/medium/low/very_low).

New files:
src/lib/fillProbability.ts - pure estimation engine, no React deps
src/hooks/useFillProbability.ts - useMemo reactive wrapper, returns null
when book data is absent
src/lib/tests/fillProbability.test.ts - 12 tests covering primary flow,
boundary cases, and failure paths

Modified files:
src/components/dashboard/SlippageTradePanel.tsx - add optional trades prop,
expand metrics grid 4->5 cols with color-coded Fill Probability stat
src/components/dashboard/DEXExplorer.tsx - thread fetched trades into
SlippageTradePanel

Invalid inputs (bad side, non-positive price/qty, NaN) return probability 0 without throwing. Existing SlippageTradePanel tests are unaffected since the trades prop is optional.

Summary

Closes #860

How was this tested?

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Security-sensitive changes

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