v19: add public API consumer and boundary tests

[flyingrobots]

Part of #712.

Problem

A v19 API reshaping can regress quietly unless consumer fixtures and boundary tests pin the intended public surface.

Scope

• Add a first-use consumer fixture that imports only root openWarp, intent, and reading plus storage subpath adapters.
• Add root export negative tests for graph/worldline/patch/hologram names.
• Add subpath typecheck coverage for storage, advanced, diagnostics, and legacy imports.
• Add browser entrypoint parity where applicable.

Acceptance

• Tests fail if forbidden substrate symbols return to root.
• Tests fail if the quick-start imports stop typechecking.
• Legacy symbols are reachable only from explicit compatibility surfaces.
• Boundary tests run in the existing local and CI test posture.

[coderabbitai]

⚠️ Possible Duplicate Issue(s)

• v19: audit and enforce root export boundary #721

🔗 Related PRs

#574 - Gate 1: enforce first-use optics honesty [merged]
#620 - Fix #282: prove architecture doc contract [merged]
#642 - Close materialization boundary proof [merged]
#654 - Clean up versioned runtime names [closed]
#660 - Replace static assertion batch 4 [merged]

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