Replace decimal.Decimal with dual int64/float64 number representation

[kaidesu]

Summary

• Removes the shopspring/decimal dependency entirely
• Replaces Ghost's single decimal.Decimal number type with a dual int64/float64 internal representation (Lua 5.3 style)
• Integer operations stay int64; any operation involving a float promotes to float64
• Division always produces a float (5 / 2 → 2.5, like Python 3)
• floor() and round(0) return integers

Motivation

decimal.Decimal uses arbitrary-precision math backed by big.Int, meaning every number literal and every arithmetic result allocates on the heap. In game engine workloads (e.g. Lumen's tilemap renderer doing ~15,000 iterations/frame), this is the dominant performance bottleneck. float64 is more than sufficient for games, and a separate decimal module can be added later for financial/scientific precision when needed.

Changes

• object/number.go — new Number struct with i int64, f float64, isFloat bool; constructors NewInt/NewFloat; arithmetic, comparison, and math methods
• ast/number.go — IntValue int64, FloatValue float64, IsFloat bool fields
• parser/number.go — parses integers with strconv.ParseInt, floats (. or e) with strconv.ParseFloat
• evaluator/ — updated number eval, postfix, prefix, index, for-in, assign
• object/ — updated string, list, map, object helpers
• library/modules/ — updated math, time, random, os
• Tests updated throughout; all passing

Test plan

• go test ./... passes

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