feat(protocol): define gateway control envelopes

openspec/changes/phase3c-apollo-input-feedback/proposal.md

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+## Why
+
+The RC5 tunnel register identifies a bounded sequenced-input flow but does not
+define typed input state or provider-to-client feedback. The native Apollo
+adapter cannot safely translate keyboard, mouse, UTF-8, controller, termination,
+rumble, or HDR state from an untyped payload.
+
+## What Changes
+
+- Define a typed, versioned gateway input envelope for keyboard, mouse, UTF-8,
+  and controller state.
+- Define bounded provider-feedback and provider-termination envelopes on the
+  existing reliable control direction.
+- Define a separate typed clipboard envelope and Server-owned per-session
+  direction, size, and rate policy so the gateway can prevent reflected loops
+  without exposing provider management material.
+- Define release semantics so gateway cleanup can emit real provider key/button
+  releases without a synthetic provider command.
+- Preserve RC5 unchanged; this change requires a new immutable Protocol version
+  after its fixtures and consumers are final.
+
+## Capabilities
+
+### New Capabilities
+
+- `gateway-input-feedback`: Typed Phase 3C gateway input, provider feedback,
+  termination, and release envelopes.
+
+### Modified Capabilities
+
+- None.
+
+## Impact
+
+- Protocol control and datagram registries, schemas, fixtures, and generated
+  Go/Rust/Swift bindings.
+- Data Plane gateway input/clipboard translation and host-feedback forwarding.
+- Connection Server mints only immutable clipboard policy in authenticated
+  provider work; it neither receives clipboard bytes nor inspects provider
+  packet payloads.