feat: v0.16 Phase 4 — domain substrate completion

[ds1sqe]

Summary

Implement the v0.16 domain substrate completion slice as part of epic #778. This phase grows the walking skeleton into the full domain substrate: Domain routing and attachment rules, focus/pending dispatch ordering, streams, coordination carriers/codecs/fallback, undo, registers, module/view-slot state, and service registry leases.

Why

The v0.16 rebuild has landed the sovereign workspace, platform floor, uapi foundation, kernel boot core, and walking skeleton. The next required phase is the substrate breadth pass that turns the one-domain skeleton into the multi-domain machinery required by the normative spec.

Proposed Change

• Complete DomainRouter/DomainTree behavior for attachments, focus chains, pending resolution, and DT17 dispatch ordering.
• Add stream substrate behavior, coordination carriers/codecs/fallback, undo, registers, module/view-slot state, and service registry leases.
• Keep kernel mechanism policy-free: domains remain registered policy identified by DomainId, with concrete behavior in domain/provider/extension crates.
• Extend conformance fixtures and the DEV2 end-to-end smoke from the walking skeleton to multi-domain text + hex-codec projection round-trips.

Acceptance Criteria

• Domain substrate chapters for the phase have zero unresolved open items or explicitly issue-tracked deferrals before implementation.
• Multi-domain fixture exercises text + hex-codec projections through the existing skeleton client.
• Depgraph probes still enforce core/ext boundary, kernel purity, DAG rules, L9 dependency sovereignty, L10 zero-std sovereignty, and L11 protocol purity.
• Same-change tests cover all new public behavior, with L12 sibling test layout and doc-tests for public API.
• ./scripts/check.sh passes.
• Coverage residue, if any, is ledgered under the existing v0.16 coverage-debt convention.

Related Work

Part of #778. Follows #797 (walking skeleton) and the #791 os-mode merge coordination.