docs: clarify view function behaviour for address(0) default allocation

Response to TRST-L-3 "Inaccurate view functions for the zero-address target"

The current behaviour of getTargetAllocation() and getTargetIssuancePerBlock()
is deliberate and regarded as correct. These functions return assigned
allocations rather than effective minting amounts. This design choice is
supported by the following reasoning:

1. **Semantically correct**: The functions answer "what allocation is assigned
   to this target?" rather than "what does this target receive?".

2. **Consistent interface**: Callers can query the default target allocation
   uniformly, regardless of whether it's address(0) or not.

3. **Accounting utility**: When address(0) is used, the returned values
   represent the unallocated/unminted portion of issuance.

4. **Separation of concerns**: getTotalAllocation() provides the effective
   system-wide allocation that excludes unmintable portions.

The documentation has been enhanced to make this behaviour explicit for the
address(0) edge case, to avoid potential confusion.

Author: RembrandtK

packages/issuance/contracts/allocate/IssuanceAllocator.sol

@@ -826,6 +826,9 @@ contract IssuanceAllocator is
 
     /**
      * @inheritdoc IIssuanceAllocationStatus
+     * @dev Returns assigned allocation regardless of whether target is address(0) or the default.
+     * @dev For address(0), no minting occurs but the allocation represents the unallocated portion.
+     * @dev For effective allocations excluding unmintable portion, use getTotalAllocation().
      */
     function getTargetAllocation(address target) external view override returns (Allocation memory) {
         AllocationTarget storage targetData = _getIssuanceAllocatorStorage().allocationTargets[target];
@@ -839,6 +842,8 @@ contract IssuanceAllocator is
 
     /**
      * @inheritdoc IIssuanceAllocationDistribution
+     * @dev Returns assigned issuance rates regardless of whether target is address(0) or the default.
+     * @dev For address(0), no minting occurs but rates reflect what would be issued if mintable.
      */
     function getTargetIssuancePerBlock(address target) external view override returns (TargetIssuancePerBlock memory) {
         IssuanceAllocatorData storage $ = _getIssuanceAllocatorStorage();