Spec + SDK: embeddable in-app WASM runtime host for downstream platform clients

[enricopiovesan]

Summary

Implement spec 057-embeddable-runtime-host: embeddable Traverse runtime WASM + uniform platform embedder SDK.

Governing spec

https://github.com/traverse-framework/traverse/blob/main/specs/057-embeddable-runtime-host/spec.md (merges via #559)

Depends on

• Approve specs 057–059: embeddable runtime, pipeline, command dispatch #559 merged (spec 057–059 approval)

Definition of Done

• Traverse runtime builds as embeddable WASM artifact
• Public embedder SDK — no private crate leakage
• scripts/ci/embedder_conformance/ per conformance.md
• Linux/CLI reference embedder runs without sidecar
• traverse-cli app validate supports execution_mode wasm|compatible

Validation

• Conformance suite passes
• Sample app executes without traverse-cli serve

Unblocks

reference-apps #113–#117

[enricopiovesan]

Downstream: reference-apps #109-#118 (Phase 3 embedded runtime + platform migrations).

[enricopiovesan]

On Traverse Project 1 (Blocked). Spec gate: #557 must be approved first.