docs(headless): close K0 and activate K1

Pin the tested Windows/Linux portability boundary, exact rollback, dependency audit, and synchronized architecture and roadmap state before starting the production single-session host.

Co-authored-by: Codex <noreply@openai.com>

docs/research/2026-07-27-slice-k0-portability-boundary.md

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+# Slice K0 portability boundary closeout
+
+## Outcome
+
+K0 completed at `aada8a37c1e933f9f0a3f41dc4f05615b01023bc`.
+The repository now has a dedicated `AcDream.Headless` executable and matching
+test project. The executable references only `AcDream.Runtime`; its transitive
+closure contains no App, UI, Silk.NET, OpenAL, Arch, ImGui, renderer, audio, or
+native-window assembly.
+
+Exact rollback:
+
+```text
+git revert aada8a37c1e933f9f0a3f41dc4f05615b01023bc
+```
+
+## Boundary
+
+- `--help` runs without constructing Runtime or loading presentation code.
+- `validate --config <path>` performs strict, case-sensitive, versioned JSON
+  validation without connecting.
+- Unknown properties, missing required fields, duplicate/empty session IDs,
+  unsupported versions, malformed files, and secret-shaped unknown fields fail
+  with deterministic exit codes.
+- Arbitrary command arguments and secret values are not reflected into
+  diagnostics.
+- Project, dependency-manifest, and loaded-assembly tests enforce the boundary.
+- Runtime's existing dependency-project test now resolves MSBuild paths on
+  Windows and Linux and remains valid under isolated artifact roots.
+
+## CI and local Linux proof
+
+`.github/workflows/headless-portability.yml` runs on Windows and Ubuntu. It
+builds Plugin.Abstractions, Core, Core.Net, Content, Runtime, and Headless,
+then tests only the presentation-free Core.Net, Content, Runtime, and Headless
+closures. Historical `AcDream.Core.Tests` App-integration fixtures remain in
+the complete-solution lane and are deliberately excluded from the no-App
+restore lane.
+
+Local Ubuntu 24.04 / .NET 10.0.302 evidence:
+
+- Plugin.Abstractions and Core build: pass.
+- Core.Net tests: 600/600.
+- Content tests: 119/119.
+- Runtime tests: 395/395.
+- Headless tests: 12/12.
+- Headless `--help`: exit 0.
+
+Exact Windows Release evidence:
+
+- solution build: pass (17 pre-existing Core-test warnings);
+- complete solution: 8,708 pass / 5 intentional skips;
+- Headless tests: 12/12;
+- Runtime tests: 395/395.
+
+## Audit
+
+- Windows-only UDP reset handling was already guarded by
+  `OperatingSystem.IsWindows()`.
+- CP-1252 registration is BCL/provider-based and Linux-safe.
+- immutable pak reading uses portable read-only memory mapping.
+- remaining wall-clock and diagnostic-call-site cleanup belongs to K2 timing
+  ownership, not the K0 process boundary.
+- no retail gameplay behavior changed and no divergence row was added.