SchemaKit: a public plugin contract, so packages contribute to the model differ and migrations SQL generator instead of replacing them #56

Problem

EF Core resolves IMigrationsModelDiffer and IMigrationsSqlGenerator as single services. A package that needs either one replaces it (ReplaceService, as this package does today). When two packages do, one silently wins (the last ReplaceService call), and the other package's declarations vanish from migrations without an error.

So this package can't be one of several migration-aware packages. That is the reason for the new name (#37): SchemaKit is the one owner of both services in the CodoMetis family, and everything else that needs them plugs in. "Schema" is the boundary. Whatever needs the differ or the migrations SQL generator belongs here or plugs in. Types, operators and query translation stay out.

Change

ROADMAP.md D10 (section 5.3) planned contributors as several DI registrations through an options extension, as an internal seam. Make it the public contract from 11.0.0:

• A contributor interface (diff a model pair into operations, render an operation per provider) and a registration that adds contributors without touching the two EF services.
• The built-in features (complex and expression indexes, exclusion constraints, typed filters, …) are contributors themselves, so the package's own features prove the contract, not only a sample.
• Dependency direction: an ecosystem package ships an optional satellite that references the SchemaKit kernel, the way CodoMetis.ValueRanges.NodaTime references its core. SchemaKit references none of them. The first prospective consumer is the ltree support in the planned CodoMetis.Hierarchies package: trigger-maintained paths, path checks, and any index options Npgsql's HasMethod/HasOperators can't express. Its type and query translation never need the contract.
• The contract is public API under the same compatibility promise as the declaration API. The wiring call is renamed along with it (UseSchemaKit(), ROADMAP.md 5.6).

Done when

• A contributor written in a test project outside the package adds an operation that migrations add, GenerateCreateScript and the pending-changes check all see, on PostgreSQL and on SQL Server.
• The built-in features are contributors.
• docs/ has a page on writing a contributor.
• It ships in 11.0.0, because the contract can't change in a minor afterwards.