docs: add the roadmap for 5.4.0 and the 6.0 family - #31

Summary

Adds ROADMAP.md at the repository root and a pointer to it in CLAUDE.md. The document is a plan, not a promise: every item is marked Decided, Proposed or Open, and the decisions that are the maintainer's alone (the name, the EF Core 10/11 route) are marked as such.

What it covers, in order:

• D0, ship first: temporal unique constraints and temporal foreign keys have no name-uniqueness validation today (only the length check), so a same-named pair scaffolds cleanly and fails at apply time. The fix is one name registry per target model across all kinds and native names, validated on the target model only.
• 5.4.0 on EF Core 10: one generic descriptor differ replacing the four copies of the diff algorithm, then a delete constraint on PostgreSQL rendered as design-time SqlOperations (a row trigger raising check_violation with the constraint name set, so the application sees a violated check constraint).
• 6.0.0, the family: rename under the reserved CodoMetis. prefix, split into a kernel and two provider packages, a plugin host built on EF's own options-extension pattern, a dialect seam, the translator moved to the kernel, snapshot format v2, the old ids deprecated with the alternate set, and the convention tests and CI that change.
• EF Core 10 and 11: one codebase multi-targeting net10.0;net11.0, 6.0.0 on EF Core 10 first, the net11.0 target in the first 6.x after GA.
• The name: a checked shortlist across three flavours, with CodoMetis.EFCore.Syntagma as the working name and the rejections recorded.
• Horizon: row-level security policies, extended statistics, domain types, deferrable constraints, trigger templates, views and stored-procedure bodies, declarative partitioning (with a feasibility verdict and a spike gate), and hierarchy support, each with the reason it was set aside and what it needs from the foundation.

Root placement is deliberate: DocumentationApiTests scans only docs/, so the roadmap may name methods that do not exist yet, while DocumentationLinkTests still checks its links.

Test plan

• DocumentationLinkTests, ClaudeMdConsistencyTests and ChangelogConsistencyTests pass locally (14 tests)
• Full suite on CI

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