Add offline collective simulation tool

[Kur0x]

Summary

• Add a standalone offline collective simulation tool under tools/collective_sim
• Model generic collective DAGs with communication-buffer validation, AllGather symbolic correctness, 1D-Clos topology timing, and SDMA/P2P/uplink calibration curves
• Add 64P and 128P AllGather examples, CLI validate/run/sweep/report, and JSON/CSV/HTML reports
• Document the no-hardware workflow and add boundary tests to prevent runtime hardware dependencies

Test plan

• PYTHONPATH=tools/collective_sim python3 -m unittest discover -s tests/collective_sim -v (Ran 42 tests, OK)
• CLI validate for 64P and 128P examples
• CLI run/sweep/report generated expected artifacts
• git diff --check 66e220f94a062f69c719ad3357e7179932f80301..HEAD