[278] Reject negative NPP allocation inputs

[Nitir4]

Summary

Reject negative leafAllocation, woodAllocation, and fineRootAllocation during model setup. For example, (-0.125, 0.25, 0.25) previously passed ensureAllocation() because the derived coarse-root fraction was positive (0.625). It now reports the supplied values through logError() and exits with EXIT_CODE_BAD_PARAMETER_VALUE (3).

Keep the existing individual upper bounds and coarse-root remainder check. Add a focused allocation regression, document the input constraints, and update the changelog.

This addresses the allocation lower-bound subtask of #278. The broader parameter, climate, and event validation work remains outside this PR. No nonfinite-input or rounding-tolerance changes are included.

How was this change tested?

• make test: all 26 registered test executables passed, including 17 allocation cases. Four active smoke scenarios (niwot, russell_1, russell_2, russell_3) matched model, event, and configuration references. The smoke summary reports 5/5 with one forced pass because russell_4 is skipped.
• make testrun: passed. The unit wrapper's PASS/FAIL table was also inspected rather than relying only on its shell exit status.
• clang-format 19.1.7: changed-line check reported no changes; the new regression file passed --dry-run --Werror.
• git diff --check: passed.

No smoke reference outputs were changed. Local clang-tidy was unavailable.

Reproduction steps

The new testAllocation.c exercises negative values in each supplied allocation, a small negative value, valid zero/interior/exact-boundary cases, individual values at or above one, and a sum above one. It checks error code 3 and preserves the supplied fractions.

make
make -C tests/sipnet/test_sipnet_infrastructure testAllocation
cd tests/sipnet/test_sipnet_infrastructure
./testAllocation

Expected final line: PASSED testAllocation (17 cases).

Related issues

• Addresses part of Add range constraints to model inputs and throw error for out of range inputs #278; this PR does not close the full issue.
• Allocation-subtask assignment request. The issue is currently unassigned; that request has not received a maintainer reply. This PR makes the small proposed change available for review.

Checklist

• Related issue and scope are listed above.
• Maintainer assignment/approval received under the contribution workflow.
• PR title includes the issue number.
• Regression tests added and registered.
• Parameter documentation updated.
• docs/CHANGELOG.md updated.
• Code checked with clang-format.

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