perf(sdk-go): cache order book price conversions

[namtran1812]

Summary

Cache the float representation of order book prices alongside their exact decimal representation.

The order book already caches parsed types.Decimal prices, but each cache hit still recomputed Decimal.Float64(). Since Float64() formats the decimal and parses it back to a float, repeated updates at the same price incurred unnecessary conversion work.

This change stores both representations in the existing bounded price cache:

• exact types.Decimal remains the source of truth for price identity, sorting, and comparisons
• cached float64 continues to serve float-returning metrics such as spread and mid
• existing cache bounds and locking behavior are unchanged

Performance

Apple M1, darwin/arm64, Go benchmark comparison (benchstat, 5 runs):

Benchmark	Before	After	Change
ApplyLevel insert	450.4 ns/op	342.9 ns/op	-23.87%
ApplyLevel update	423.5 ns/op	315.4 ns/op	-25.53%
LiveOrderBook IngestDiff	947.8 ns/op	695.8 ns/op	-26.59%

Allocations:

Benchmark	Before	After
ApplyLevel insert	11 → 9 allocs/op	-18.18%
ApplyLevel update	11 → 9 allocs/op	-18.18%
LiveOrderBook IngestDiff	27 → 23 allocs/op	-14.81%

The latency geomean decreased 25.34% across the measured paths.

Validation

• go test ./websocket/orderbook -count=10
• go test -race ./websocket/orderbook -count=1
• go test ./... -count=1
• go vet ./...
• git diff --check

Added regression coverage verifies that cache hits preserve float-returning metrics and exact price-level identity for prices that cannot safely use float64 as their identity.

[nostradamus-bot]

Nostradamus Risk Rating — Low

PR touches only packages/sdk-go/websocket/orderbook/book.go, a pure in-memory market-data caching struct in the Go SDK with no auth, credential, financial-transaction, or trust-boundary involvement; the change is a performance optimization caching Float64() conversions alongside the existing Decimal source of truth, with regression tests explicitly covering float-precision edge cases — no security surface is affected.