refactor(superdoc): close strict-null cluster in SuperDoc.js (SD-2867)

[caio-pizzol]

SD-2867 strict-null cluster, grouped PR. Closes all 40 TS2532 / TS18048 / TS2533 errors in packages/superdoc/src/core/SuperDoc.js using one consistent set of patterns. No more per-error stacking.

Three mechanisms cover the cluster:

1. Class field declarations for store-state fields that #initVueApp populates synchronously inside #init (superdocStore, commentsStore, highContrastModeStore, app, pinia, readyEditors, pendingCollaborationSaves). Declared without initializers because the constructed instances aren't available at field-init time; this matches the existing pattern for users / version / whiteboard. Closes ~25 sites where TS thought the field could be undefined because the assignment lived in a separate method.

2. InternalConfig cast at sites that read this.config.documents, this.config.modules, or this.config.layoutEngineOptions after #init has normalized them. InternalConfig (in core/types/index.ts) gains four required-field overrides (documents, modules, user, layoutEngineOptions) representing the runtime invariants #init always produces. Public Config is unchanged; only the internal augmentation type tightens. Closes ~13 sites.

3. Make the runtime invariants real: #init now explicitly normalizes this.config.documents = this.config.documents || [] and this.config.layoutEngineOptions = this.config.layoutEngineOptions || {} after normalizeTrackChangesConfig. The modules || {} and user defaults already existed; this completes the set so every InternalConfig cast site has the value it claims. Note: normalizeTrackChangesConfig already populates layoutEngineOptions internally, so the explicit guard here is belt-and-suspenders rather than a fresh fix - it makes the invariant legible at the #init boundary instead of relying on a side effect of an unrelated helper.

Public surface is unchanged. The SuperDoc .d.ts emits the same config: Config field; the new class fields emit with their precise ReturnType<typeof useStoreFn> types (no any leaks). InternalConfig is not re-exported through the package entry, so consumers don't see the tightening.

Total errors in SuperDoc.js (under probed // @ts-check) drop from ~130 to ~102. Remaining categories (TS2339 cascade, TS2345, TS2322) land in subsequent SD-2867 grouped PRs per the updated ticket plan.

Verified:

• pnpm --filter superdoc check:jsdoc → 3 gated files clean
• pnpm --filter superdoc build:es → no FAIL-level findings
• pnpm --filter superdoc audit:declarations → no FAIL-level findings
• node tests/consumer-typecheck/typecheck-matrix.mjs → 33 passed, 0 failed

Must stay the same:

• Public Config shape (no required fields added)
• Consumer view of superdoc.config (still typed as Config)
• Consumer view of superdoc.superdocStore / commentsStore (still emit Pinia store types; SD-2915 tracks the deep any cleanup separately)

Review: confirm InternalConfig changes don't leak into public .d.ts (they shouldn't since the type isn't re-exported); confirm the #init normalization changes aren't observable to consumers (no behavior change for configs that already pass these fields, and the documents: [] default already existed via the class-field initializer for the no-source case).

[linear]

SD-2867 checkJs gate: enroll SuperDoc.js (Phase 2)

[superdoc-bot]

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[superdoc-bot]

🎉 This PR is included in superdoc-sdk v1.9.0