docs: describe governed multi-surface target

[abrichr]

Summary

• replace temporary experimental/research/design-partner substrate framing in the flagship README with the governed multi-surface target architecture
• describe browser structure, Windows UI Automation, native macOS/Linux accessibility, and RDP/Citrix remote-display adapters under one uniqueness/identity/policy/postcondition/effect/halt contract
• keep required repository/package lifecycle labels and workflow-specific deployment qualification rather than making blanket certification claims
• align the Cloud/Desktop/customer-controlled product-surface description without mixing this copy change into the secure pairing PR

Verification

• 31 passed, 1 skipped in the launcher test suite
• git diff --check passed