EF migration generation: documentation

[jeremydmiller]

Part of the EF Core migration generation epic — tracking issue #371.

Documentation for the whole feature. All code samples must be mdsnippets sourced from compilable test projects (repo convention — no inline hand-typed samples).

New pages (under docs/efcore/)

• Migration generation overview — what it is, when to use it vs. Weasel's native db-patch/db-apply flow; the generated artifacts (migration classes, stub context, snapshot file) and what each is for.
• Getting started — db-ef-migration add → dotnet ef database update walkthrough with a Marten-style IDatabase.
• Coexistence guide — mixed EF Core + Marten/Polecat/Wolverine applications: multi-context setup, the relocated history table, --context usage, and the single-owner rule (a table is managed by exactly one migration stream; excluding critter-stack-managed tables from the app context via IsTableExcludedFromMigrations).
• EF Core projections — the round-trip story: projection DbContext → MapToTable → IDatabase → generated migrations include projection tables automatically; ownership guidance.
• Adopting existing databases — db-ef-migration baseline / history backfill.
• Limitations & raw-SQL fallbacks — partitioning, functions/procedures, sequences, rich index options emitted as Sql() blocks; no dotnet ef migrations add/remove against the stub context; EF 9 PendingModelChangesWarning suppression explained.

Updates to existing docs

• docs/efcore/migrations.md — position the two directions (Weasel-applies vs. EF-artifact generation) side by side.
• Computed columns documented in the relevant table-modeling pages (PostgreSQL + SQL Server), since Weasel gains that capability in this epic.
• CLAUDE.md / EFCORE_IMPROVEMENTS.md — update capability lists and remove closed gaps.

Downstream (tracked in their own repos, not here): short consumption pages for Marten, Polecat, and Wolverine.