Correct 20-contract §19's list of /authoring exports (#366)

Carried over from
[#316](#316),
which was closed as superseded. This was the one part of that branch
that had not landed independently.

## What was wrong

`20-contract.md` §19 described `@the-running-dev/game-engine/authoring`
as exporting:

> generic content-registry builder, the story-graph source builder, the
shared adventure builder and its migration helper …

Two things are inaccurate against `src/engine/src/authoring.ts`:

- The subpath exports `buildCampaign` — a **campaign** builder, from
`core/registry/build.ts`. `buildContentRegistry` is not exported from
`/authoring` at all, so a reader following §19 would go looking for a
surface that is not there.
- The adventure exports are three, not two: `buildAdventureCampaign`,
`createAdventureSource`, `migrateV1AdventureState`. The sentence named
the builder and the migration helper and dropped the source factory.

`docs/docs/guide.md` carried the same drift in its own words ("the
content-registry builder, the story-graph and adventure source builders
and their migration helpers") and is corrected to match.

## What changed

- `design/20-contract.md` §19 — the canonical edit, two lines.
- `docs/docs/engine/04-core.md` — regenerated by
`./build/ConvertTo-HumanDocumentation.ps1`, not hand-edited.
- `docs/docs/guide.md` — the affected paragraph corrected and the design
digest re-stamped with `-StampGuide`.

Wording only. No type, signature, or export changes — the code was
already right; the contract was describing it wrongly.

## Note on how the guide was regenerated

`/make-human-docs` overwrites `guide.md` in full with no partial
regeneration. For a two-noun correction in one paragraph of an 841-line
file, a full rewrite would have produced a large unrelated diff on a
branch scoped to one sentence, so the affected paragraph was corrected
in place and the digest re-stamped. The gate that matters —
`Test-Documentation.ps1`'s guide-digest check — passes either way, but
the choice is recorded here rather than left to be inferred from the
diff.

## Gates

| Gate | Result |
|---|---|
| `./build/Test-Documentation.ps1` | pass — drift check across 18
generated engine pages, compatibility pointers and the guide; 125
Markdown files |
| `npm run typecheck` | pass |
| `npm run lint` | pass |
| `npm test` | pass — 79 files, 1173 tests |
| `git diff --check` | clean |

**Did not run:** `./docs.ps1 -BuildOnly`, the production Docusaurus
build. `docs.ps1` is not installed in this checkout, so it could not be
run locally — the *Verify Documentation Build* check on this PR is the
gate that covers it, and it should be read there rather than assumed
from the above.

Author: The-Running-Dev

design/20-contract.md

@@ -1539,8 +1539,8 @@ them, so a rename breaks old data.
 
 The package root is the runtime contract. `@the-running-dev/game-engine/authoring` is the
 separate author-time contract for repositories that own campaign source. It exports the
-generic content-registry builder, the story-graph source builder, the shared adventure
-builder and its migration helper, portable serialization and manifest digests, and
+generic campaign builder, the story-graph source builder, the shared adventure builder with
+its source factory and its migration helper, portable serialization and manifest digests, and
 replay-runner types/functions. It is deliberately a
 subpath: a runtime host must not import authored campaign source merely to play published
 portable JSON.

docs/docs/engine/04-core.md

@@ -1511,8 +1511,8 @@ them, so a rename breaks old data.
 
 The package root is the runtime contract. `@the-running-dev/game-engine/authoring` is the
 separate author-time contract for repositories that own campaign source. It exports the
-generic content-registry builder, the story-graph source builder, the shared adventure
-builder and its migration helper, portable serialization and manifest digests, and
+generic campaign builder, the story-graph source builder, the shared adventure builder with
+its source factory and its migration helper, portable serialization and manifest digests, and
 replay-runner types/functions. It is deliberately a
 subpath: a runtime host must not import authored campaign source merely to play published
 portable JSON.

docs/docs/guide.md

@@ -3,7 +3,7 @@ sidebar_position: 1
 sidebar_label: Developer Guide
 ---
 
-<!-- design-digest: 4cd4099867b95bef280462a7c52e0449d702b7b163867a2e29fc9fad84c3eab9 -->
+<!-- design-digest: 5a2c5126ec1504b8ee04110ce91843f08484d01673a4055edae77b9dc7e1b0dd -->
 
 > Generated from `design/` by `/make-human-docs`. Do not edit by hand — edit the
 > design docs and regenerate. `/reconcile` reports when this has gone stale.
@@ -130,14 +130,14 @@ manifest hosts fetch. This engine stays the authority for deterministic mechanic
 validation, and portable hydration.
 
 The package root is for runtime hosts. A separate subpath, `/authoring`, is for repositories that
-own campaign source: it exports the content-registry builder, the story-graph and adventure source
-builders and their migration helpers, portable serialization and manifest-digest functions, and
-replay-runner types. Import from `/authoring` only when writing or publishing campaign content —
-a runtime host must never import authored campaign source merely to play already-published
-portable JSON. `toPortable` and the manifest-resolution digest are `/authoring`-only;
-`fromPortable` is root-only; the portable-campaign digest function is exported from both, because
-both a runtime host verifying fetched content and a publishing pipeline digesting source before it
-ships need it.
+own campaign source: it exports the generic campaign builder, the story-graph source builder, the
+adventure builder with its source factory and its migration helper, portable serialization and
+manifest-digest functions, and replay-runner types. Import from `/authoring` only when writing or
+publishing campaign content — a runtime host must never import authored campaign source merely to
+play already-published portable JSON. `toPortable` and the manifest-resolution digest are
+`/authoring`-only; `fromPortable` is root-only; the portable-campaign digest function is exported
+from both, because both a runtime host verifying fetched content and a publishing pipeline
+digesting source before it ships need it.
 
 A handful of frozen campaigns still ship as package-root exports for compatibility. They are
 regression fixtures, not a publication source, and a later breaking release removes them from the