Walkthrough: one booking app from request to response, as a sample that CI builds and runs #418

Problem

Each guide covers one boundary: binding and JSON in docs/serialization.md, mapping and queries in docs/efcore.md, the exclusion constraint in docs/efcore.md#reject-overlapping-rows-with-an-exclusion-constraint. None shows them working together in one application, and none says where a value changes on the way through: canonical form on construction, microsecond truncation when it's stored, infinity at the extremes, the literal on the wire.

A walkthrough written as prose alone would go stale, because nothing compiles its snippets.

Change

• A sample application that runs. A minimal API booking service, using:

  • a requested stay bound from the query string and from a JSON body;
  • validation;
  • storage with an exclusion constraint against overlapping bookings per room;
  • room features as a StringSet, filtered with @>;
  • availability as the bookings' RangeSet, its gaps found with Except;
  • a JSON response.

  It adds the constraint with explicit SQL in a migration, so it works on both lines (ComplexIndexes can't be installed beside the EF Core 11 packages until SchemaKit ships, SchemaKit 11.0.0-preview.1: the first slice of the new API, on EF Core 11 / Npgsql 11 under the new ids EFCore.ComplexIndexes#38).

• CI runs it and checks what it says. It runs against a live PostgreSQL and asserts on the HTTP responses and the translated SQL, never on the exit code alone. The natural home is a fifth consumer in test/consumer-smoke-test.sh. That way it compiles outside the repository against the packed packages, as a reader's project would, and Directory.Build.props doesn't apply.

• docs/walkthrough.md, in the plain-docs voice, quotes excerpts from the sample and links to the full source. Each step says what changes at that boundary, or that nothing does. A convention test checks that every quoted excerpt still matches the sample's source, so they can't drift apart.

• Claims. The walkthrough's promises go into claims.json. If heldBy can't name the smoke consumer, an integration test holds them.

• Both lines. It lands on main first and is cherry-picked with -x into release/10.x, with 10.x package versions.

Done when

• The sample builds and runs in CI on every PR on both lines, and fails when an answer it prints changes.
• docs/walkthrough.md is linked from the README and docs/getting-started.md, and from the docs site's navigation once that exists.
• The excerpt check fails when a quoted excerpt is edited without changing the sample's source: seed it and check that it fails.