Fix conceptual seams in Foundry anatomy

[jmchilton]

What changed

• Clarify that the extension surface contains domain-specific decisions, some required and some optional, rather than an identical list of slots.
• Align corpus-grounding with the current upstream-authority and licensing principle: citations, authored derivatives, pinned snapshots, and working caches are all legitimate when their source and refresh posture are explicit.
• Make self-documentation part of the reader-facing knowledge base by naming glossaries, design records, kind documentation and examples, and generated catalogs.
• Separate the shared independent-evaluation discipline from the domain-specific mechanism used to provide a verdict.
• Replace the “inherited essentially for free” claim with a more credible account of reduced reinvention plus required integration and verification.
• Turn the summary table into direct mappings between what stays consistent and what each domain decides.

Why

These are conceptual consistency fixes discovered while reviewing the Build one sequence. Landing-page names, page structure, and the broader prose rewrite are intentionally excluded so this PR can land before that editorial work begins.

The larger question of whether validation should become a layered guiding principle remains open in #41; this PR only makes the current policy internally consistent.

Impact

Readers get a more accurate boundary between the reusable Foundry substrate and domain-owned decisions, without changing the Model or introducing a new principle.

Validation

• npm test — 22 tests passed
• npm run build — wiki-link check passed; 29 pages built and indexed
• git diff --check