docs: capture test snapshot boundaries and family anchors

[bingran-you]

Summary

• document the source-snapshot packaging boundary that hides runner, CI, coverage, and fixture-corpus assets
• add concrete test framework anchors for visible file families, test/preload.ts, sharding, and single-file script entrypoints
• tighten fixture docs around fixtures/ layout, configurable roots, and naming families

Verification

• npx -p first-tree first-tree verify