refactor: move buildable .NET source into src/, update CI/doc paths

Groups the repo root into src (buildable source), docs (already existed),
and everything else (db/, scripts/, tests/, deploy/ - deployment/ops/test
assets that aren't compiled, already organized as their own folders). CI
now only needs src/ to build: dotnet restore/build/test/publish all point
at src/TaxBaik.sln, src/TaxBaik.Web/, src/TaxBaik.Proxy/.

- git mv every project (Domain, Infrastructure, Application,
  Application.Tests, Web, Web.Client, Proxy) and TaxBaik.sln into src/ as a
  unit, so relative ProjectReference/.sln paths stay valid unchanged.
- .gitea/workflows/deploy.yml: 6 dotnet restore/clean/build/test/publish
  invocations now point at src/. db/migrations and scripts/ stay at root
  (deploy_gb.sh and browser-e2e.yml only touch published output and the
  deployed URL, not source paths - verified, no changes needed there).
- scripts/validate_admin_render.sh: admin render-mode file paths now
  src/TaxBaik.Web.Client/...
- scripts/validate_kst_timestamps.sh: dropped deploy.sh from its target
  list - that script was removed in the prior cleanup commit (dead, no
  CI workflow referenced it) but this validator still expected it to exist.
- CLAUDE.md, docs/ENGINEERING_HARNESS.md, docs/ADMIN_PATTERN_CRITIQUE_WBS.md:
  updated project-structure diagram, dotnet run/build commands, and grep
  targets to the new src/ paths (also fixed a pre-existing stale path in
  ADMIN_PATTERN_CRITIQUE_WBS.md that still said TaxBaik.Web/Components/Admin
  from before that ever moved to TaxBaik.Web.Client).
- Added a Repo Root harness rule + Architecture Guardrail entries: new files
  belong under src/docs/tests/scripts/db/deploy, not loose at root; temp
  work stays outside the repo (or under a gitignored .scratch/) and is
  never committed.

Verified locally: dotnet build/test src/TaxBaik.sln (26/26 tests), and all
three scripts/validate_*.sh pass against the new layout.

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

src/TaxBaik.Application/Utils/BusinessDayCalculator.cs

@@ -0,0 +1,75 @@
+namespace TaxBaik.Application.Utils;
+
+public static class BusinessDayCalculator
+{
+    private static readonly HashSet<DateOnly> HolidayDates = new()
+    {
+        // 2026
+        new DateOnly(2026, 1, 1),
+        new DateOnly(2026, 2, 16),
+        new DateOnly(2026, 2, 17),
+        new DateOnly(2026, 2, 18),
+        new DateOnly(2026, 3, 1),
+        new DateOnly(2026, 3, 2),
+        new DateOnly(2026, 5, 5),
+        new DateOnly(2026, 5, 25),
+        new DateOnly(2026, 6, 6),
+        new DateOnly(2026, 8, 15),
+        new DateOnly(2026, 8, 16),
+        new DateOnly(2026, 8, 17),
+        new DateOnly(2026, 9, 24),
+        new DateOnly(2026, 9, 25),
+        new DateOnly(2026, 9, 26),
+        new DateOnly(2026, 10, 3),
+        new DateOnly(2026, 10, 4),
+        new DateOnly(2026, 10, 5),
+        new DateOnly(2026, 10, 9),
+        new DateOnly(2026, 12, 25),
+
+        // 2027
+        new DateOnly(2027, 1, 1),
+        new DateOnly(2027, 2, 6),
+        new DateOnly(2027, 2, 7),
+        new DateOnly(2027, 2, 8),
+        new DateOnly(2027, 2, 9),
+        new DateOnly(2027, 3, 1),
+        new DateOnly(2027, 3, 2),
+        new DateOnly(2027, 5, 5),
+        new DateOnly(2027, 5, 13),
+        new DateOnly(2027, 6, 6),
+        new DateOnly(2027, 8, 15),
+        new DateOnly(2027, 8, 16),
+        new DateOnly(2027, 9, 14),
+        new DateOnly(2027, 9, 15),
+        new DateOnly(2027, 9, 16),
+        new DateOnly(2027, 10, 3),
+        new DateOnly(2027, 10, 4),
+        new DateOnly(2027, 10, 9),
+        new DateOnly(2027, 10, 10),
+        new DateOnly(2027, 10, 11),
+        new DateOnly(2027, 12, 25),
+        new DateOnly(2027, 12, 26)
+    };
+
+    public static DateOnly GetEffectiveDueDate(DateOnly dueDate)
+    {
+        var effectiveDate = dueDate;
+        while (!IsBusinessDay(effectiveDate))
+        {
+            effectiveDate = effectiveDate.AddDays(1);
+        }
+
+        return effectiveDate;
+    }
+
+    public static int GetDday(DateOnly dueDate, DateOnly? referenceDate = null)
+    {
+        var today = referenceDate ?? DateOnly.FromDateTime(DateTime.Today);
+        var effectiveDueDate = GetEffectiveDueDate(dueDate);
+        return effectiveDueDate.DayNumber - today.DayNumber;
+    }
+
+    public static bool IsBusinessDay(DateOnly date)
+        => date.DayOfWeek is not DayOfWeek.Saturday and not DayOfWeek.Sunday
+           && !HolidayDates.Contains(date);
+}

src/TaxBaik.Application/Utils/VersionInfo.cs

@@ -0,0 +1,7 @@
+namespace TaxBaik.Application.Utils;
+
+public class VersionInfo
+{
+    public string Version { get; set; } = "unknown";
+    public string Built { get; set; } = "unknown";
+}