Spec: prefer one local model; add vision as an open decision

The host-sharing item now records the operator's preference: one local
model for every local role, with agents tagging in and out of the host
as the fallback if no single model fits. A new item records vision as a
requirement (OpenSCAD renders, later video files) and what the text-only
agent needs before any vision work.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/Architecture-Build-Spec.md

@@ -1423,15 +1423,34 @@ The rest resolve when reached, not now.
   don't address this: they cap one key's throughput rather than share the host,
   and know nothing about which model is loaded. (Measured 2026-09-28: they only
   throttled the benchmark, and the token limiter leaked until LiteLLM restarted.)
-  Options, simplest first:
-  - *One local model for every local role.* No swaps at all. It needs one model
-    good enough both as the dev executor and for ops' Home Assistant tool calls.
-  - *Two models co-resident*, if VRAM allows (llama-swap groups): for instance a
-    small tool-caller for ops beside the executor model. It costs context window.
-  - *A scheduler in front of the host* that batches each model's requests to keep
-    swaps rare. The most work; only if neither of the above fits.
-  Decide once the executor benchmark and the ops evaluation show whether one model
-  can serve both roles.
+  - **Preferred: one local model for every local role.** No swaps at all. It
+    needs one model good enough as the dev executor, for ops' Home Assistant tool
+    calls, and for vision (next item). The executor benchmark and the vision and
+    ops evaluations decide whether one exists.
+  - **Fallback: agents tag in and out.** If no single model fits, the agents
+    coordinate their use of the host rather than interleave: an agent holds the
+    host for a stretch of work on its model, then hands it over, so a swap happens
+    per hand-over rather than per request. Two models co-resident (llama-swap
+    groups), where VRAM allows, can reduce how often that's needed. How the
+    hand-over works (a lease, a queue, or priority for ops alerts) is only worth
+    designing if the preferred option fails.
+- **Open: vision (A10, A15).** Needed for `nisse-dev`'s OpenSCAD work, where the
+  model must look at a rendered part to check it against the design, and for
+  video-file management planned later (which agent owns it is not yet
+  decided). The local candidates on the
+  4090 host accept images, and video as sampled frames, but the agent is
+  text-only: canonical events carry strings, and neither renderer can send an
+  image. Needed before any vision work:
+  - a way for a tool to return an image, e.g. an OpenSCAD render or a frame from
+    a video file;
+  - image content in the canonical event model, rendered as OpenAI image parts
+    and Anthropic image blocks, so delegation to a frontier model keeps it;
+  - storage that keeps the durable log (A2) small, e.g. a reference and hash
+    rather than the image bytes;
+  - image tokens counted in the context budget; video frames multiply them.
+  Which local model is good enough at seeing is measured separately from
+  tool calling: an evaluation with rendered OpenSCAD parts carrying seeded
+  mistakes, and sample video frames.
 
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