feat: add BigDecimal type to CVM (bounty #5)

[aglichandrap]

Summary

Implements arbitrary-precision decimal arithmetic for the CVM numeric tower, fulfilling bounty #5.

Changes

• CVMBigDecimal.java: Wraps java.math.BigDecimal, extends ANumeric

  • Exact decimal arithmetic (add, sub, multiply, divide)
  • Type promotion: Decimal op Integer → Decimal, Decimal op Double → Double
  • CAD3 encoding: tag 0x1a + VLQ signed scale + VLQ byte count + unscaled bytes
  • Canonical if and only if value has meaningful fractional digits (integers stay as Long/BigInteger)

• Decimal.java: Type class for CVMBigDecimal registered as Types.DECIMAL

• Core.java: Registers bigdecimal (code 260) constructor and bigdecimal? (code 261) predicate

• CAD3Encoder.java: Decodes tag 0x1a → reads VLQ scale, VLQ byte count, unscaled bytes → constructs CVMBigDecimal

• Juice.java: costNumeric() charges proportional to unscaled byte length (same as BigInteger)

• RT.java: ensureBigDecimal() conversion from any numeric type

• Tag.java: BIG_DECIMAL = 0x1a

• Symbols.java: BIGDECIMAL, BIGDECIMAL_Q

Design Decisions

• Code 260 for bigdecimal constructor, 261 for bigdecimal? predicate
• Tag 0x1a for wire format (between BIG_INTEGER=0x19 and DOUBLE=0x1D)
• Canonicality: Only decimals with fractional digits are canonical; integers always reduce to CVMLong/CVMBigInteger
• Division: Non-terminating decimal division throws ArithmeticException (preserves exactness)
• Numeric dispatch: CVMBigDecimal handles Integer operands directly; falls back to Double for CVMDouble operands

Fixes #5