Split test/sandbox-unit.sh into focused test files #5

Parent: #3

Problem

test/sandbox-unit.sh is 1245 lines, exceeding the 400-line limit.

Solution

Split into focused test files:

• test/unit/encrypt-decrypt.sh
• test/unit/model-discovery.sh
• test/unit/config-generation.sh
• test/unit/guards.sh
• test/unit/validation.sh

Update test/helpers.sh and Makefile accordingly.