Release 10.3.0: date the changelogs, tag v10.3.0, carry the sections to main #470

Change

These are the release steps once every in-scope PR of #408 has merged into release/10.x, as #365 did for 10.2.0. Version is already 10.3.0 (#426). The last fixes, #464 (PR #468) and #465 (PR #469), merged on 2026-10-25, so the release commit to check is release/10.x at 7c01380 or later.

1. Check before the tag. All of the following must hold on the release commit, with every fix merged, so the fixes are checked together rather than one PR at a time:
   • Every CI lane is green: PostgreSQL 16 and 18, on the EF Core floor and on the newest-patch legs, plus both smoke-test modes. These remain the gates.
   • dotnet run tools/oracles.cs seeds re-proves locally that every seed is caught (AGENTS.md, workflow step 4).
   • No further discovery round runs before the tag. The outside pre-tag review of 15085f9 already ran, and its two findings are fixed (A Compare over ranges in a query runs another method, or runs in SQL, when it isn't IComparer's own #464, Booking sample answers 500 for a null or missing stay #465).
2. Date the five changelogs. Set ## [10.3.0] — yyyy-mm-dd in all five, in the commit that gets tagged. No test or workflow checks the date.
3. Tag v10.3.0 (annotated, like v10.2.0) on that commit. The release workflow publishes the packages, and the nuget environment approval is the maintainer's. This run is the first to exercise the 10.0.402 SDK pin in release.yml (CI: pin the .NET 10 SDK to 10.0.402 while aka.ms links resolve to a host that answers 409 #461).
4. Check that no 10.x-only issue is left open. Closes #n acts only on main. Every sub-issue of Epic: 10.3.0 #408 is closed already, so this step only re-checks.
5. Carry 10.3.0 to main in one PR, as chore: carry the 10.2.0 sections to main (#365) #413 did for 10.2.0. It can't build until 10.3.0 is on nuget.org, because the baseline bump restores it.
   • Copy the [10.3.0] sections into main's five changelogs, verbatim.
   • Add ## Changed answers in 10.3.0 to main's docs/migration.md. It links the two subsections under Migration from v10.x and says that the last two items of that list also shipped in 10.3.0 (Epic: 10.3.0 #408, "Release").
   • Make whatever changes the conventions then require on main: PackageValidationBaselineVersion, main's SECURITY.md row and the walkthrough sample's package version.
6. Open 10.4.0 or close the line for additions on release/10.x, as Open 10.3.0 on release/10.x #426 did after 10.2.0: Version, the baseline, the SECURITY.md row and the sample move to 10.3.0.

Done when

• v10.3.0 is on nuget.org for all four packages.
• GitHub's Latest release is still the highest stable version.
• Epic: 10.3.0 #408 is closed with a summary.
• main carries the 10.3.0 sections.