feat(protocol): negotiate display and native input

openspec/changes/phase-3d-client-display-input/proposal.md

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+## Why
+
+The Phase 3D native client must request a bounded display mode and send
+provider-neutral absolute-pointer and high-resolution scroll input. The current
+wire contract exposes only policy-selected stream dimensions and rejects both
+new input kinds.
+
+## What Changes
+
+- Add one bounded `DisplayMode` object and optional display fields to session
+  request/session/manifest contracts while keeping control wire version 1.
+- Register `display.request.v1`, `input.absolute.v1`, and `input.scroll.v1` so
+  every new behavior is explicitly negotiated.
+- Extend VGI1 with exact absolute-pointer and signed scroll bodies while
+  preserving all existing kind encodings.
+- Require a new immutable Protocol release candidate after deterministic
+  Go/Rust/Swift generation and conformance pass; existing tags remain unchanged.
+
+## Capabilities
+
+### New Capabilities
+
+- `session-display-mode`: Optional feature-gated requested and effective display
+  mode disclosure for native session creation and manifests.
+
+### Modified Capabilities
+
+- `gateway-input-feedback`: Add negotiated provider-neutral absolute-pointer and
+  high-resolution scroll kinds to the existing sequenced input grammar.
+
+## Impact
+
+- JSON Schema, feature registry, VGI frame documentation, fixed conformance
+  fixtures, generated Go/Rust/Swift bindings, and strict validators.
+- Connection Server session persistence/allocation and Data Plane input
+  translation after consumers pin the immutable Protocol candidate.
+- Protobuf remains unchanged because no observed consumer uses it for these
+  JSON control or VGI payload paths.