Fix #501: Add PrecedenceConstrainedScheduling model

[zazabap]

Summary

• Add PrecedenceConstrainedScheduling satisfaction problem model (Garey & Johnson A5 SS9)
• Unit-length tasks with precedence constraints, m processors, deadline D
• Satisfaction problem: does a valid schedule exist?

Fixes #501

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.62%. Comparing base (d309549) to head (f4dd731).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #626      +/-   ##
==========================================
+ Coverage   96.61%   96.62%   +0.01%     
==========================================
  Files         218      220       +2     
  Lines       29352    29499     +147     
==========================================
+ Hits        28357    28504     +147     
  Misses        995      995              

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[zazabap]

Implementation Summary

Changes

• src/models/misc/precedence_constrained_scheduling.rs — New model: satisfaction problem for scheduling unit-length tasks on m processors by deadline D with precedence constraints
• src/unit_tests/models/misc/precedence_constrained_scheduling.rs — 12 unit tests: basic, evaluation (valid/invalid/edge cases), brute force, serialization
• src/models/misc/mod.rs — Register module and re-export
• src/models/mod.rs — Add to top-level re-exports
• problemreductions-cli/src/dispatch.rs — Add load_problem and serialize_any_problem dispatch arms
• problemreductions-cli/src/problem_name.rs — Add lowercase alias resolution

Deviations from Plan

• Added input validation: num_processors > 0 and deadline > 0 assertions when num_tasks > 0 (quality review finding)
• Used 0-indexed time slots {0, ..., D-1} instead of issue's 1-indexed {1, ..., D} — standard representation choice

Open Questions

• Complexity string uses brute-force bound "deadline ^ num_tasks". No well-known refined exact algorithm exists for general PCS with variable m.
• Paper entries (display-name + problem-def) deferred to separate PR.