feat(common): overflow-safe deposit->token reconciliation scaling (#5)

[real-venus]

feat(common): overflow-safe deposit → token reconciliation scaling

Closes #5

Solution

contracts/common/src/scaling.rs (pure #![no_std], no new dependencies),
built on the exact 256-bit mul_div_floor from crate::weighted_rate:

• reconcile_tokens(deposit_amount, asset_precision) -> Result<u128, ScaleError>
  • validates ASSET_PRECISION ∈ [1, 10¹²] → Err(InvalidPrecision);
  • holds deposit × 10¹⁸ in full 256-bit precision and divides exactly — it
    never wraps;
  • returns Err(Overflow) when the mathematically-correct token amount exceeds
    u128::MAX, instead of a silently wrapped value;
  • floor rounding — never mints more than the deposit backs (error < 1 base
    unit).
• scale(amount, scale_factor, precision) — generic variant.
• is_valid_precision, is_safe_deposit, MAX_SAFE_DEPOSIT — conservative
  early-reject helpers for callers that want them.

Files

• contracts/common/src/scaling.rs (new)
• contracts/common/src/lib.rs (register module)
• contracts/docs/specs/reconciliation-scaling.md (new)