refactor: consolidate diff engine into pkg/diff using gotextdiff

Move diff algorithm and Kubernetes diff engine from internal/ to pkg/ so
flux-manifest-preview can import it as a library.

Key changes:
- Replace custom Myers diff (~312 lines) with github.com/hexops/gotextdiff
- Move internal/diff/diff.go → pkg/diff/myers.go (thin gotextdiff wrapper)
- Move internal/engine/diff.go → pkg/diff/engine.go (ObjectRef-based API)
- Move internal/engine/reorder.go → pkg/diff/reorder.go
- Move all remaining internal/engine/* → pkg/engine/* (public pipeline)
- Unify on ObjectRef instead of unexported resourceKey

This eliminates ~1000 lines of custom diff code and aligns both projects
on a single, battle-tested diff library.

chore: remove easyjson dependency

Update go-openapi dependencies to versions that no longer require
easyjson as a transitive dependency:

- go-openapi/jsonpointer v0.21.0 -> v0.22.4
- go-openapi/jsonreference v0.20.2 -> v0.21.4
- go-openapi/swag v0.23.0 -> v0.25.4

Removes github.com/mailru/easyjson and github.com/josharian/intern.

v0.1.0 — initial public release