chore(deadcode): ratchet down the 85 pre-existing unused exports/types (follow-up to #607)

Follow-up to #607 (PR #673).

After #673 re-greened the dead-code ratchet, the knip baseline holds at **85** — all of it pre-existing drift, no longer masked by the devDep/false-positive noise:

| category | count |
|---|---|
| \`exports\` | 44 |
| \`types\` | 41 |

## Why this matters

The ratchet job (\`check:deadcode-ratchet\`) is **advisory / non-blocking** today (\`continue-on-error: true\`). Its stated milestone is to *"flip to blocking once the baseline is driven to zero."* Because it is non-blocking, roughly 7 genuine unused export/type regressions drifted into \`main\` unnoticed inside the +13 that #607 reconciled. Every merge that adds dead code under a non-blocking gate pushes the flip-to-blocking milestone further out.

## Ask

Chip the 85 down over one or more scoped PRs, lowering \`knip-baseline.json\` \`count\` in the same PR each time to lock in the gain (per the baseline's own guidance). Removing unused exports/types requires per-symbol analysis — some may be public API surface or referenced dynamically — so this is deliberately **not** a single bulk-delete.

Reproduce the current list:

\`\`\`
node_modules/.bin/knip --reporter json | jq '[.issues[] | {file, exports: (.exports // [] | map(.name)), types: (.types // [] | map(.name))}] | map(select((.exports|length)>0 or (.types|length)>0))'
\`\`\`

Once the count reaches 0, drop \`continue-on-error: true\` from the ratchet CI job to make it blocking.

まず knip-baseline.json を読み、提供されている knip コマンドを実行して、未使用の export と型 85 件を調査します。各シンボルの参照を確認してから、そのシンボルを所有するソースファイルを変更し、対象を絞ったクリーンアップごとに baseline の件数を更新します。件数が 0 に達し、ratchet CI ジョブから continue-on-error: true が削除されれば完了です。

索引モデルが issue の本文から書いたものです。