Codex/update semaprax readme

[wsdt]

Semantic change

Describe the meaning changed, affected stable IDs, completion-matrix rows, and trust-boundary impact.

Evidence

• Canonical source round-trip remains deterministic.
• Agent graph/transaction behavior is covered where relevant.
• Success and stable diagnostic cases are tested.
• Native and Wasm behavior agree where runtime meaning changed.
• scripts/quality.sh or its platform-equivalent commands pass.
• Architecture, roadmap, changelog, and completion evidence are accurate.
• Public syntax/schema/protocol/CLI/manifest changes preserve a compatibility fixture or include a version bump and migration note.
• No capability, unsafe boundary, network access, or generated cache was introduced implicitly.

Platforms exercised

List concrete hosts, simulators/devices, artifacts loaded, and behavior observed. A build without execution is not runtime evidence.