refactor(content): split the moon catalogue reader out of build preparation

[apresmoi]

What changed

• The moon-catalogue reader moves out of build preparation into its own site/moon-catalogue.mts; object-children.mts and preparation both read it, so production no longer imports build preparation.
• Preparation keeps the search-based ordering and the named-moon eligibility policy.
• site/moon-catalogue.test.mts and a preparation characterization test pin the old and new readers to the same catalogue and validation errors.
• The S3-4 plan row is removed from docs/site-architecture/, its move is recorded, and .github/site-refactor.json declares S3-4 moon catalogue with layout: "changes".

Checks

Local lane against origin/main (1d1973e39e), ci.mts --head <worktree> --base <base>:

• L1/L3 comparison: exit 0. 13 differences, all inside the closure (3 modules, 2 chunks); emitted html, js and css bytes equal; nothing outside the closure; counts equal (46450 files, 35804 modules, 35030 chunks, 9892 pages).
• L2 server answers: identical for preview (351), netlify (349) and cloudflare (346).
• L7 counted build guard: PASS, no increases, no added or removed routes.
• pnpm check:architecture: ARCHITECTURE_OK; site plan tables match.
• Touched tests (moon-catalogue, prepare-body-moons characterization, object-children characterization, body-moons): 12 of 12 pass.
• pnpm typecheck: pass. pnpm lint: 0 errors.

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