Specify declared native DING delivery

[myobie]

Summary

• resolve DQ2 with an explicit native delivery selector
• retire the rendered-screen-only assumption for maintained native harnesses
• keep legacy ding behavior unchanged and require new binaries to validate transport values and mutual exclusion
• state the true pre-change behavior: a valid deliver-only agent gets no delivery instead of the wrong transport
• require Doctor to report active agents that declare no transport
• preserve the durable inbox and fail-closed retry boundary across transports

Validation

• git diff --check
• nix flake check --print-build-logs could not run because Nix is not installed on this host
• local cargo fmt --all --check and strict Clippy expose pre-existing current-main toolchain drift in unchanged Rust files
• local cargo test --all-features --locked reaches a pre-existing catalog_diff failure in unchanged code; all preceding suites pass

This PR contains only the standalone VRS closure. Parser, Doctor, and harness implementations follow in separate PRs. No merge is requested before Nathan reviews the exact head.

[myobie]

Implementation discovery found a compatibility blocker in this draft contract: current main and the retained predecessor both ignore unknown agent child nodes, so they silently ignore deliver instead of rejecting it. The PR remains draft while CoS chooses the migration boundary. Do not review the old-binary rejection sentence as settled yet.

[myobie]

Compatibility correction is now pushed at exact head 5e52199. The VRS no longer claims that pre-change binaries reject deliver. It states the verified behavior: those binaries ignore the new child, so a valid deliver-only declaration has no delivery rather than legacy screen delivery. The contract now requires new-binary validation plus a Doctor report for missing transport.

[myobie]

Final VRS alignment is now at exact head 2db8a8c: the ontology defines DING as a delivery signal rather than only terminal input, and the Agent Spec implementation map records the mutually exclusive ding / deliver selectors. The compatibility correction remains unchanged.

[myobie]

Folded into the reviewable specification replacement #242 at exact head b5debb736b5fd416eac7294849ed816242bd5625. The replacement preserves this draft’s relevant history and has exact-head hosted Nix green. Closing this superseded draft; its branch is intentionally retained.