feat: implement strangler-fig-demo Session 1 (backend + smoke script)

Builds the four-seam, three-write-path reference demo backend: case-framework
(seam D stand-in), legacy-backend/frontend (SQL Server, seams A/B/C targets),
and new-backend (Domain/Application/Infrastructure.*/Api implementing the
source resolver, take/release-ownership, write-through translator, and owned
assessment flow), wired together via docker-compose with a plain placeholder
frontend standing in for the Angular portal until Session 2.

All 11 Architecture.Tests pass and scripts/smoke.sh passes end-to-end against
a fresh `docker compose up`, covering acceptance criteria 1-3 and 7-22.

Fixes two real domain bugs found only once the stack ran for real: the BSN
eleven-proof checksum trivially passes all-zero digits, and the adoption
mapper silently treated a partial legacy address as absent instead of failing
loudly. Also fixes several environment-specific integration issues (rootless
Podman/SELinux bind-mount permissions, a buildah NuGet layer-caching bug,
SqlClient's invariant-globalization incompatibility, and an nginx path-prefix
mismatch for the legacy frontend).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

new/src/New.Application/WriteThrough/WriteThroughResult.cs

@@ -0,0 +1,28 @@
+namespace New.Application.WriteThrough;
+
+/// <summary>
+/// A single portal-shaped field error, after the write-through translator has
+/// mapped a legacy `veld`/`code`/`melding` triple. <see cref="Detail"/> only
+/// carries the raw legacy message, and only for codes the translator did not
+/// recognize - see the translator's class-level comment on why it must never
+/// invent business meaning for a code it doesn't know.
+/// </summary>
+public sealed record PortalFieldError(string Field, string Message, string? Detail = null);
+
+public enum WriteThroughOutcomeKind
+{
+    Success,
+    ValidationFailed,
+    Conflict,
+    NotFound,
+}
+
+public sealed record WriteThroughOutcome(WriteThroughOutcomeKind Kind, IReadOnlyList<PortalFieldError>? Errors = null)
+{
+    public static readonly WriteThroughOutcome Success = new(WriteThroughOutcomeKind.Success);
+    public static readonly WriteThroughOutcome Conflict = new(WriteThroughOutcomeKind.Conflict);
+    public static readonly WriteThroughOutcome NotFound = new(WriteThroughOutcomeKind.NotFound);
+
+    public static WriteThroughOutcome ValidationFailed(IReadOnlyList<PortalFieldError> errors) =>
+        new(WriteThroughOutcomeKind.ValidationFailed, errors);
+}