Add requireAligned() and requireMinStep() convenience guards #93

Motivation

When using lossy quantization with encode(), consumers often need to enforce input constraints upstream to prevent silent precision loss. Two patterns recur:

1. Reject values below step size to prevent encode() silently returning 0 (floor-to-zero). The library has no error for this case; consumers must define their own (e.g., ContributionBelowMinimum). This is the main gap: a BelowMinStep(uint256 value, uint256 stepSize) error would let consumers revert with a standardized library error instead of inventing domain-specific ones.

2. Reject non-aligned values to guarantee lossless per-entry storage. The library already has NotAligned and isAligned(), but no standalone guard function that performs the check-and-revert. Consumers currently write if (!scheme.isAligned(value)) revert NotAligned(value, scheme.stepSize()); manually. A requireAligned(Quant, uint256) convenience would reduce boilerplate.

Proposed API

/// @notice Thrown when a non-zero value is smaller than the scheme's step size.
error BelowMinStep(uint256 value, uint256 stepSize);

/// @notice Reverts with BelowMinStep if value > 0 and value < stepSize.
///         Zero is allowed (represents empty/uninitialized state).
function requireMinStep(Quant self, uint256 value) internal pure;

/// @notice Reverts with NotAligned if value is not a multiple of stepSize.
function requireAligned(Quant self, uint256 value) internal pure;

BelowMinStep is the genuinely missing error definition. requireAligned() is a convenience wrapper around the existing NotAligned error and isAligned() function.

Context

These patterns came up while integrating the library into octant-v2-core, where ProperQF packs per-project quadratic funding data into a single storage slot. We ended up defining ContributionBelowMinimum and WeightNotAligned as custom errors in the consumer contracts, but BelowMinStep is generic enough to live in the library.