docs: explain test declaration scope

[kvlonge]

Why

Scoped macro tests now inherit production declaration visibility, and grouped declaration containers require a concrete owner below resource roots.

Changes

• Explain authored-root _sqlbuild/ rejection.
• Document expected-model declaration unions for tests.
• Distinguish scenario enum/constant relationships from test macro access.
• Explain tested-macro owner context, lexical test helpers, and mock boundaries.
• Update Scope Explorer relationship terminology.

Verification

• Cross-checked against compiler, cache, placement, and rule behavior
• Public tree and outgoing-history hygiene scans
• Independent documentation review and finding-focused follow-up