Complete the first-cast guidance

[jmchilton]

What changed

• add the missing Astro-stack stage for casting one real Mold to one target
• define the composition boundary among the reference contract, corpus index, Kind layouts,
  target declaration, hooks, cast findings, and provenance
• make fixed companion membership Kind-owned throughout the glossary, Kind catalog, planning guide,
  and implementation checklist
• distinguish fixed Kind companions from additional per-note companions on explicitly open layouts
• document the pre-cast vertical: reader, Kind, Mold, reference validation, and rendering remain
  useful before @galaxy-foundry/cast is installed
• give the first bundle a concrete behavioral validation matrix without pinning corpus totals

Why

The implementation guide described the cast package but jumped from authoring Molds directly to the
domain's external check. It also gave two conflicting answers about companions: Kind dispositions
were described as enforced, while another section required fixed membership to be repeated per
note.

The corrected boundary is:

• the Kind owns fixed layout and companion disposition
• cast consumes that same layout through kindLayouts
• bundled fixed companions travel automatically with a resolved note
• foundry-only and cast-input companions cannot be reclassified by note metadata
• per-note companion lists are additions available only to explicitly open Kind layouts

The new casting stage also records a useful adoption boundary: a reader-only instance uses the
plain reference-contract loader and should reject uninterpreted cast: configuration; the composed
cast loader is adopted only when the first real Mold is ready to compile.

Dependency

The Kind-owned cast companion API is introduced by jmchilton/foundry-lib#95. Keep this PR in draft
until that contract is merged and published.

Validation

• npm run check:links — 21 files, no broken links
• npm test — 22 tests pass
• npm run typecheck — zero errors; one existing unused-variable hint
• npm run build — 28 pages built and indexed by Pagefind

The build retains the existing CSS optimizer warning about a Google Fonts @import order; this
change does not touch site styles.