architecture: complete Codectx ambient HUD steering v0

[greygoody]

Purpose

Complete steering workcell #11 without implementing the compositor, Codectx App, runtime integration, or new public schemas.

Step 2: architecture

• inventory current implementation and connected work;
• define host-neutral lens, compositor, projection, attention, move, surface, Codectx App, and host boundaries;
• define source authority, freshness, contradiction, visibility, priority, and state-lifetime rules;
• freeze current v1 compatibility surfaces;
• gate host selection on accepted runtime contracts.

Step 3: migration and handoffs

• split implementation into host-neutral semantic/compositor proof and later runtime-hosted integration;
• define prerequisites, proof gates, stop conditions, and compatibility policy;
• provide downstream implementation handoffs;
• keep semantic retrieval experimental.

Boundary

Documentation only. No Rust, crate layout, packet schema, CLI, focus, Runseal, legacy, or Orb-manifest behavior changes.

Evidence

Contribution-only evidence is recorded on branch data/codectx-steering-v0-evidence in greygoody/orb-eval under:

data/contributions/v1/codectx-steering-2026-06-30/steering-workcell/codectx-ambient-hud-v0/

Proof

GitHub Actions workflow proof run #49 passed on head 65d0b8446da52d1fce4f8647dc2bc03925cee194.

Downstream workcells

• Workcell handoff: implement host-neutral Codectx compositor vertical slice #13: host-neutral compositor vertical slice, ready after this PR merges;
• Blocked handoff: integrate Codectx App with an accepted host #14: runtime-hosted Codectx App integration, blocked on Workcell handoff: implement host-neutral Codectx compositor vertical slice #13 and an accepted host contract.

Closes #11.