Clarify the two accountability boundaries

[jmchilton]

What changed

• rewrites The Two Assets around two explicit questions: where a package came from, and what decided that work performed with it was acceptable
• introduces an early two-lane diagram separating the production boundary (source → cast → package + provenance) from the trust boundary (result → independent check → verdict)
• distinguishes provenance carried beside a cast from the domain capability that performs external validation
• tightens the Galaxy and Statistical Genomics examples and states the latter's current implementation status accurately
• replaces absolute claims with narrower guarantees about traceability, declared validation coverage, and non-self-certification

Why

The previous page treated provenance and the external check as if they were two similar things an artifact carries. They answer different accountability questions at different points in the lifecycle. Leading with those boundaries makes the argument easier to understand and keeps the page aligned with the Model and Guiding Principles.

Validation

• npm test — 22 tests passed
• npm run typecheck — 0 errors (one pre-existing unused-index hint)
• npm run build — 28 pages built; wiki-link check clean
• git diff --check