docs(openspec): archive phase 3d display input

2026-08-10 23:09:58 +07:00
parent 408d4f9cc3
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+## Context
+
+The JSON control schema is the generated-binding authority for broker requests
+and manifests, while VGI1 is the provider-neutral input payload. Both contracts
+are strict: old decoders reject unknown response fields and old gateways reject
+unknown VGI kinds. Phase 3D therefore needs optional fields plus explicit
+feature negotiation rather than a wire-version or protobuf change.
+
+## Goals / Non-Goals
+
+**Goals:**
+
+- Preserve legacy JSON shapes when display negotiation is absent.
+- Generate optional referenced objects correctly in Go, Rust, and Swift.
+- Define exact, bounded, cross-language absolute-pointer and scroll bytes.
+
+**Non-Goals:**
+
+- Server clamp policy, database persistence, or provider translation.
+- Live display renegotiation, provider packet exposure, or protobuf changes.
+
+## Decisions
+
+- Keep control wire version 1 and gate additions with exact feature IDs. This
+  avoids changing every legacy request while allowing strict clients to demand
+  the accepted display field.
+- Use one reusable `DisplayMode` with the existing provider-policy dimension
+  bounds. Optional referenced objects become Go pointers so `omitempty` is real;
+  Rust and Swift retain their existing optional generation.
+- Encode absolute pointer as four big-endian u16 values and scroll as two
+  big-endian i16 values in VGI1. Viewport coordinates are self-contained and
+  provider-neutral; provider scaling stays outside Protocol.
+- Leave protobuf unchanged because the observed broker and VGI consumers use
+  JSON and fixed byte frames, not generated protobuf messages.
+
+## Risks / Trade-offs
+
+- [Old strict consumers reject new response fields] → emit them only after
+  `display.request.v1` negotiation.
+- [New VGI kinds reach an old or unadvertised gateway] → reject before provider
+  translation unless the matching input feature is active.
+- [Generated Go optional values serialize as zero objects] → require pointers,
+  omission tests, and clean second generation.
+
+## Migration Plan
+
+Freeze a new never-reused Protocol RC after full verification. Server and Data
+Plane then pin that exact tag together. Legacy sessions omit all new fields and
+continue using existing input kinds.
+
+## Open Questions
+
+None at the Protocol boundary; policy clamp and Apollo translation remain
+consumer-owned work.