draft roadmap (#166)

Author: wpbonelli

docs/dev/map.md

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+# FloPy 4 development roadmap
+
+## Phase 1: Minimum Viable Product
+
+Core functionality. Shortcuts are OK.
+Read/write input files, run simulations, get/set data.
+Initial deliverable for alpha testing (USGS, Deltares).
+Alpha testers include Joeri, Huite, and the core MF6 team.
+Release via `pip install ` from github URL of our development sandbox.
+Tentative timeframe: July.
+
+- [x] DFN spec to TOML
+- [x] draft object model
+- [x] unified IO framework
+- [ ] MF6 input file parser
+- [ ] MF6 input file writer
+- [ ] code generation from spec
+- [ ] minimal docs (quickstart, etc)
+
+## Phase 2: Minimum Marketable Product
+
+Refactor to make production-ready.
+Achieve rough feature-parity with 3.x.
+Adopt features from e.g. `imod-python`.
+Incorporate feedback from alpha testing.
+Beta testers include alpha testers and additional volunteers from USGS and Deltares.
+Release via `pip install` from github URL of our development sandbox.
+Tentative timeframe: by end of year?
+
+- [ ] reimplement object model (maintainability over magic)
+- [ ] structured xarray index for topology/geometry-aware selections
+- [ ] xarray accessors for cross-cutting concerns (plot, export)
+- [ ] xugrid integration for UGRID-compliance (DIS/DISV grids)
+- [ ] more extensive docs (converted from old flopy examples)
+- [ ] IO optimization (tuning, laziness/concurrency, etc)
+- [ ] comprehensive logging and error handling
+- [ ] validation framework, model checks
+
+## Phase 3: Rollout
+
+Integrate the product with the existing repository.
+Evaluate feature-parity and fill any remaining gaps.
+Release via standard channels (PyPI, Conda).
+Dial 3.x down to maintenance mode.
+
+- [ ] implement 3.x adapters, compare tests/examples
+- [ ] finalize 3.x maintenance plan and 4.x release plan
+- [ ] finalize MF6 compatibility policy and codegen tooling