Enforce scheduling constraints

[MohammadSyed18]

What changed

• enforce deadlines as hard scheduling boundaries
• reject overlapping availability blocks
• report whether unscheduled work is deadline-, dependency-, or capacity-constrained

Why

The scheduler scored lateness but could still place work after its deadline, producing plans that looked complete but violated task constraints. Plans now remain feasible and explain omissions.

Validation

• cmake --build build
• ctest --test-dir build --output-on-failure
• demo planner run and git diff --check