Rewrite ROADMAP.md for the SchemaKit spin-off: D3–D11 decided #41

Rewritten 2026-10-19. The 2026-10-18 version of this issue planned a rename at 10.0.0 on EF Core 10 and two lines under the new ids. That plan is superseded by the spin-off (#36, #37).

Set the decision rows of ROADMAP.md to:

• D3 → Decided: rename at the EF Core 11 boundary, as a new package, not at 6.0/10.0 on EF Core 10.
• D4 → Decided: CodoMetis.EFCore.SchemaKit, with .PostgreSQL and .SqlServer. It names the boundary: SchemaKit owns the model differ and the migrations SQL generator, and other packages plug into it. Rejected: Syntagma (every listing would need the tagline to explain it) and DataKit (unspecific, and taken by a DataKit.* family on nuget.org).
• D5 → Decided: three packages, unchanged.
• D6 → Decided: the old ids stay supported on EF Core 10, with fixes only after 5.5.0. They are deprecated, with SchemaKit as the alternate, only when EF Core 10 leaves support.
• D7 → Decided: two lines. EFCore.ComplexIndexes 5.x on EF Core 10 (release/5.x), SchemaKit 11.x on EF Core 11 (main). A fix that applies to both goes to main first and is cherry-picked with -x.
• D8 → Decided: SchemaKit ships previews during the EF Core 11 RCs and 11.0.0 when it's ready.
• D9 → Decided: SchemaKit starts at 11.0.0 and tracks the EF Core major. The old ids continue as 5.x.
• D10 → Decided: the plugin host is a public contract (SchemaKit: a public plugin contract, so packages contribute to the model differ and migrations SQL generator instead of replacing them #56), plus an ownership check (SchemaKit: fail at startup when another package has replaced the model differ or the migrations SQL generator #57).
• D11 → Decided: snapshot format v2 ships in 11.0.0 (SchemaKit: snapshot format v2 and CodoMetis: annotation keys ship in 11.0.0; the 5.x format is read forever (D11) #58).

Then rewrite the prose to match:

• The features planned for 5.5.0 (4.1–4.3) move to SchemaKit 11.x.
• Section 1 and the "why one stream and not two" paragraph.
• Section 8 (EF Core 11).
• Section 9: the net11.0 edge is resolved by the EF Core major guard (Refuse EF Core 11 at registration: EFCore.ComplexIndexes serves EF Core 10 only #55).
• Section 10 (sequence).