Phase 3: land embedded runtime architecture spec and governance updates

[enricopiovesan]

Summary

Add docs/embedded-runtime-plan.md and update constitution, runtime docs, README, AGENTS.md, quality standards, and design language to define Phase 3: embedded in-app WASM runtime as the production target (HTTP sidecar demoted to dev-only).

Why

All platform clients must eventually bundle the Traverse host. Current docs describe only the Phase 1/2 sidecar model, which contradicts the product architecture.

Depends on

• Product decision: embedded runtime is production target (confirmed)

Blocked by

• None (documentation)

Definition of Done

• docs/embedded-runtime-plan.md exists with platform matrix, multi-capability showcase design, ticket index
• Constitution Principle V + portability updated for Phase 3
• docs/traverse-runtime.md distinguishes dev sidecar vs embedded production model
• README + AGENTS.md blocked-work section references Phase 3
• docs/design-language.md Zone 1 updated for embedded runtime status

Validation

• bash scripts/ci/repository_checks.sh passes
• Docs cross-link consistently; no doc still claims sidecar is the only/production model