Architecture Decision Records (ADR)

Short, durable notes for why the codebase looks the way it does. They complement narrative docs (architecture.md, refactoring_roadmap.md).

ADR	Title
0001-record-architecture-decisions.md	Why we keep ADRs
0002-gradient-operators-fd-vs-spectral.md	FD vs spectral operators (direction)
0003-time-integrator-interface.md	Time integrator interface contracts
0004-execution-layer.md	Execution layer for 0.2 — minimal homegrown, no Kokkos
0005-fft-interface.md	FFT interface — split host and device transforms
0006-precision-policy.md	Precision policy — template on RealType, instantiate double
0007-decomposition-splitter.md	Decomposition splitter — in-repo, HeFFTe as FFT-only dependency
0008-io-formats.md	I/O and checkpoint formats for 0.2

New ADRs: copy the template sections from 0001, use the next number, and add a row here.