build: slots instead of placeholders, and the first worded run

Fallback words read like finished copy, and nemotron handed the front
door straight back. The prompt now shows every string as a slot saying
what belongs there; a slot echoed back counts as no answer and keeps
the fallback.

runs/contractor-build-2026-09-21: nemotron-3.5-lightning:30b-a3b, the
model that produced no loadable site free-form, words a complete
contractor site in 130 s. Verifier green by construction; held-out
personas route 8/8 with descriptions and 8/8 on headings alone
(qwen3:8b as the visitor). "What happens next" is still 0/2 on the
front door: no heading or description says who reads a submission.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01L4jrCgLiKKHAEFuZUJjckH

README.md

@@ -8,6 +8,38 @@ series for every role, the state machines behind them, and a desk per
 handling role. No software is written for the business; the only thing
 produced is content, and every line of it is held to a verifier.
 
+## Build: structure from the plan, words from the model
+
+```
+portal-trainer build --plan plans/contractor.yaml --out out/nordvik \
+    --lint question_lint --llm "MODEL=nemotron-3.5-lightning:30b-a3b engines/ollama.sh"
+```
+
+A `plan.yaml` is a needs spec that also says what each form collects.
+Everything structural is derived from it with no model involved: the
+state machines (read off `stages`: walk the line, be turned down at
+any point, succeed only from the end, pass over `skippable` stages),
+the front door, a followup page per need, a gated directory per group
+with its desk and the pages only that group may open, and every button
+the machine allows. The verifier accepts the result by construction.
+
+The model writes words only, one page at a time, into a JSON object
+whose shape is fixed. Its strings are merged over built-in fallbacks
+key by key, so a bad reply costs wording, never a working site.
+
+| Same contractor, same local model (nemotron 30B-A3B on ergo) | Free-form `generate` | `build` |
+|---|---|---|
+| A site the verifier accepts | never, in 6 rounds and 16 min | first try, 130 s |
+| Held-out personas, with descriptions | - | 8 of 8 |
+| Held-out personas, headings only | - | 8 of 8 |
+
+For comparison, a Sonnet-class model writing everything free-form
+scored 8 of 10 on headings only and silently dropped a stage the owner
+had described. `build` cannot drop a stage: it never decides one.
+
+`generate` (below) remains for turning a plain-words intake into a
+first plan; what it should hand over is the plan, not the pages.
+
 ## How it works
 
 ```