feat(backport): counter-based RNG for deterministic JER smearing

[alefisico]

Summary

The current JER smearing in CorrectedJetsFactory uses numpy.random, which produces different random numbers depending on how events are partitioned across chunks. This means the same jet can get different smearing corrections depending on the processing setup, making results non-reproducible.

This PR adds an opt-in counter-based RNG (CBRNG) using the Squares algorithm (arXiv:2004.06278) that generates deterministic random numbers based on per-jet physics quantities (event number, phi, eta). The same jet always gets the same random number regardless of partitioning, chunk size, or parallelization strategy.

This is the v0.7.x backport of the same feature on master #1535 . The key difference is that CBRNG calls are executed eagerly with numpy.asarray() instead of being wrapped in awkward.virtual, since v1's virtual arrays can produce stale data on sliced arrays.

This developement was created by @chuyuanliu in our framework, and I am just putting it in the central coffea because I consider it important for other users.

Key points:

• New event_number parameter on build() — when provided, uses CBRNG; when omitted (None), falls back to the legacy RNG. Fully backwards compatible.
• Per-jet 128-bit counter: [event_number(64), phi_bits(32) << 32 | eta_bits(32)]
• Eager evaluation to avoid awkward v1 virtual pitfalls

Files changed

• coffea/jetmet_tools/CorrectedJetsFactory.py — added event_number/seeds parameters to build(), CBRNG path alongside legacy RNG
• coffea/jetmet_tools/cbrng.py — new module implementing the Squares CBRNG algorithm
• coffea/jetmet_tools/correctionlib_adapters.py — minor fix
• tests/test_correctionlib_adapters.py — added test_corrected_jets_factory_cbrng verifying determinism and partition independence

Test plan

• test_corrected_jets_factory_cbrng — verifies same jets produce same smearing across calls
• All 16 existing tests pass unchanged (no regressions)

[alefisico]

Note: This PR depends on #1522 (correctionlib adapters backport). The diff currently includes changes from that branch — once #1522 is merged into backports-v0.7.x, this PR will automatically show only the CBRNG commit.