Release v1.6 — faster migration engine + survey solver

[ovitrac]

Summary

Brings the public tree to v1.6.

Engine

• patankar/migration: pass the constant sparse Jacobian to the BDF integrator (cold-start speed-up); C0eq guardrail and unit-C0eq resume fix.

Survey

• New survey/chain_engine.py — unified real-units N-layer × K-step migration solver core.
• New survey/physical_query.py — physical-primary survey query (evaluates the real-units chain at the actual time-prior points; no Fo-surface interpolation).
• New survey/utils/ — cf_at_user_grid safe CF extraction.
• survey/workers.py — canonical equilibrium helpers _cfeq_from_sol / _eq_knee (single source of truth).
• Refreshed shared survey modules (io, models, priors, aggregation, survey, README).

Versioning

• Package 1.50 → 1.6 (setup.py, pyproject.toml, patankar/__init__.py, README).
• Survey module 0.1.0 → 0.4.0 (+ CHANGELOG [0.4.0]).
• New TODO.md.

Cache

• Full PubChem / ToxTree cache sync (PubChem 3,009 → 11,131; ToxTree 4,373 → 12,738), isolated in its own commit 660bf58 for review.

Validation

• pytest tests/ — 52 passed.
• All new modules import cleanly.

Notes

• Two commits: code/doc (1cae682) and cache (660bf58).
• The new engine modules ship import-safe but are reachable only through the multilayer/two-step survey path (tracked in TODO.md).
• HTML documentation is not regenerated here (handled by the dedicated doc pipeline).