fix(seo-graph-core): return GraphEntity from piece builders

[schlessera]

Stacked on #54. Until that merges, this PR shows both commits; the diff here reduces to the GraphEntity change once #54 lands. Per-PR review of just this change: compare the second commit.

Problem

assembleGraph<T extends GraphEntity>(pieces: readonly T[]) requires each piece to satisfy GraphEntity, whose @type is required:

// types.ts
export interface GraphEntity {
    '@type': string | readonly string[];
    '@id'?: string;
    [key: string]: unknown;
}

But every builder is declared to return Record<string, unknown>, which has no @type. So the pattern documented in the core README and AGENTS.md —

assembleGraph([buildWebSite(...), buildArticle(...)]);

— fails tsc / astro check under strict mode:

Property '@type' is missing in type 'Record<string, unknown>' but required in type 'GraphEntity'.

Callers must cast as GraphEntity[]. The same bite hits the aggregate / createSchemaEndpoint mapper, whose type is already (entry) => ReadonlyArray<GraphEntity> — builder outputs don't satisfy it without a cast.

Fix

Widen the return type (and the local piece) of all eight builders to GraphEntity:

buildWebSite, buildWebPage, buildArticle, buildBreadcrumbList, buildImageObject, buildVideoObject, buildSiteNavigationElement, buildPiece.

Every builder already constructs its object with a literal @type, so this is a pure type change — no runtime difference. buildPiece's public overloads already require @type on the input, so its result is a GraphEntity; the implementation signature stays wider and carries one local as GraphEntity (documented inline).

No changes needed in astro-seo-graph — aggregate already expects GraphEntity; this just lets callers satisfy it without a cast.

Test

Added an assembleGraph test that composes real builder outputs directly (no cast). Because pnpm typecheck covers the test files, it doubles as a compile-time regression guard — it would not have type-checked under the old Record<string, unknown> returns.

Verification

pnpm build && pnpm typecheck && pnpm test && pnpm format:check all green locally (core 60 tests, +1 new; 475 total).

changeset: minor (widens public return types; non-breaking).