refactor(objects): the prepared object format constant and world-context summary decoders move into objects

[apresmoi]

What moved

packages/renderer/src/prepared-data/object-format.ts           -> packages/objects/src/prepared-data/object-format.ts
packages/renderer/src/prepared-data/world-context-summary.ts   -> packages/objects/src/prepared-data/world-context-summary.ts
                                                                + packages/objects/src/prepared-data/world-context-summary.test.ts

• PREPARED_CSS_OBJECT_FORMAT and the table decoders expandWorldContextSummary / expandWorldSystem are exported from @cssearth/objects.
• Renderer and bake import them from there; the renderer re-exports of the constant are gone.
• The format string stays cssearth-css-object@5. The moved code differs only in its guard import.
• parsePreparedWorldContextSummary and its types stay in the renderer: they share parseContext with the camera and presentation contracts.

Baseline

• Package cycles: 0 -> 0
• Cycle-closing folder edges: 6 -> 6 (23 imports baseline, unchanged)

Checks

• Clean build: packages/*/dist deleted, pnpm install --frozen-lockfile, pnpm build:packages, build-ci.mts lint
• pnpm check:architecture
• pnpm typecheck
• objects, renderer and bake package tests; site asset-origin, world-billboards and prepared-world-context tests
• pnpm check:body-references
• Documentation link audit

Follows the pull request rules.