Decide the casing of the codelist allowed_outputs key

Found while verifying adopter-facing configuration documentation against the v0.21.0 release.

## Inconsistency

The codelist artifact grammar uses `allowed_outputs` (snake_case):

```yaml
id: urn:gov:example:codelist:region-map
version: '1'
entries: {SOURCE-A: REGION-NORTH, SOURCE-B: REGION-SOUTH}
allowed_outputs: [REGION-NORTH, REGION-SOUTH]
```

Every other multi-word key across the bundle and runtime grammars is camelCase (`assuranceProfile`, `bundleDirectory`, `maximumFileBytes`, `holderBoundBatchMaxSize`, `acquisitionCapabilities`, ...). `allowed_outputs` is the lone snake_case multi-word key an adopter writes, and it is easy to mistype as `allowedOutputs`, which the closed grammar rejects as an unknown key.

## Suggested fix

Pre-1.0 this is still changeable: either rename to `allowedOutputs` for consistency, or, if the codelist artifact grammar is considered frozen with the Version 1 contract, document the exception explicitly where the artifact is described so adopters stop tripping on it.

まず、issue に記載されている codelist artifact grammar と adopter 向けの設定ドキュメントを見つけ、次に allowed_outputs を bundle grammar と runtime grammar にある他の複数語キーと比較します。完了とは、大文字・小文字の方針が記録され、関連する grammar references が一貫して更新されるか、例外が adopter 向けに明示的に文書化されていることを意味します。

索引モデルが issue の本文から書いたものです。