v0.21.0

Release 0.21.0

Breaking changes:
- drop support for Python 3.9
- `Snaps` no longer inherits from `Steps`
- `Rprofs` is now an ABC implemented by both `RprofsInstant` (replacing
  the previous `Rprofs`) and `RprofsAveraged`
- `Fields` is no longer a `Mapping`
- remove `Fields.geom`, use `Step.geom` instead
- remove `ciover`, `cfover`, `advth` profiles that were highly
  problem-specific

Improvements:
- support Python 3.13, 3.14
- unexpectedly reaching the end of an xmf file now raises a `EndOfXml`
  error instead of `RuntimeError`
- tighten type annotations
- `StagyyData` is a frozen dataclass, and several of its properties are
  now cached properties
- `Steps`, `Snaps`, `StepsView`, `Step`, `Fields`, `Geometry`,
  `RprofsInstant`, `RprofsAveraged`, `Tseries, `Refstate`, and `Tracers`
  are also frozen dataclasses
- leverage `rich` to format the output of some commands

Fixes:
- `FieldXmf` and `TracersXmf` work correctly for sequential runs
- `RprofsAveraged` supports variable time steps
- plate distribution plot is fixed with matplotlib 3.10

Internal:
- respect PEP639 for license specification
- switch to src layout
- use `uv_build` as build backend
- use `uv` to publish release