Public API for third-party / custom projections #242

Context

The projection-genericity effort (#239, #240, #241) made projections pluggable internally: ProjectionDefinition (strategy) + ProjectionRegistry map each Projection enum case to its transform math, all below the public API surface. During that work we decided explicitly not to expose the protocol publicly ("Keep internal" decision, made after reviewing the trade-offs).

What this issue would add

An option for users of the library to register custom projections (e.g. a local grid CRS or a projection the library hasn't shipped) without forking:

• A public protocol (or struct-based descriptor — probably preferable; public protocols are sticky API commitments, see Swift API design guidelines) exposing:
  • forward/inverse relative to the EPSG:4326 pivot
  • validExtent / worldBoundingBox metadata (would need a proper struct, not the current tuple in the internal protocol)
  • kind: ProjectionKind capability classification
• A registration API (dictionary-backed lookup replacing/customizing the current eager dictionary in ProjectionRegistry)
• Likely also a way for custom SRIDs to appear as Projection values — which conflicts with the current closed enum: Int design (would need a raw-value-backed case or a restructure)

Considerations / prior decisions

• The current internal-first design was deliberate: nothing in shipping projections requires a public API, and pre-registry phases pinned semantics (noSRID verbatim copy, pivot routing) that a public seam would have to preserve
• .swiftformat/style conventions and strict concurrency apply to any public surface
• If pursued, design the descriptor now while the internal protocol is still small; retrofitting is more expensive

Related

• Effort overview issue series: Introduce ProjectionKind and capability-based projection dispatch #237/Route projections through a ProjectionDefinition registry #238/Add EPSG:3395 and EPSG:32662 projections #239/Registry-driven WKT CRS matching and coder/tile test coverage #240/Add UTM zone projections (EPSG:32601–32660, 32701–32760) #241 (phase PRs)