[AZ-500] Cycle 4 Step 9: new-task .NET 10 migration

Closes Step 9 (New Task) of cycle 4. AZ-500 spec defines the
.NET 8 -> .NET 10 migration (TFM bump on 9 csprojs, global.json
SDK pin to 10.0.0, both Dockerfiles + run-tests.sh + woodpecker
to mcr.microsoft.com/dotnet/*:10.0, Microsoft.AspNetCore.* and
Microsoft.Extensions.* to the 10.x line, Serilog.AspNetCore to
10.x or documented 8.0.3 fallback, plus arch.md + AGENTS.md doc
sync). Closes the cycle-3 perf-harness leftover via AC-5
(bootstrap smoke after migration).

Also logs the cycle-4 perf-leftover replay attempt that
discovered the host-SDK / project-SDK mismatch and rolls the
state file from cycle 3 -> cycle 4 (Step 9 done -> Step 10
ready).

Co-authored-by: Cursor <cursoragent@cursor.com>

_docs/_autodev_state.md

@@ -2,13 +2,13 @@
 
 ## Current Step
 flow: existing-code
-step: 9
-name: New Task
-status: not_started
+step: 10
+name: Implement
+status: in_progress
 sub_step:
-  phase: 0
-  name: awaiting-invocation
-  detail: "CYCLE 3 COMPLETE. Step 17 (Retrospective) wrote retro_2026-05-12_cycle3.md + structure_2026-05-12_cycle3.md + 3 new ring-buffer lessons in LESSONS.md. Verdict: first cycle with net-negative architecture delta (-3 Medium-or-above resolved, 0 new Medium); 100% review pass rate (5/5 batches); 5 of 6 tasks completed first attempt; all prior-retro actions translated to closed work. Loop back to Step 9 (New Task) for cycle 4 per existing-code.md § Re-Entry After Completion."
+  phase: 1
+  name: prerequisite-checks
+  detail: ""
 retry_count: 0
 cycle: 4
 tracker: jira