## Why

The canonical terminal-receipt requirement contradicts the fixed VGF1 grammar and every generated validator, so RC8 cannot provide an executable cross-language contract for gateway-owned terminal closure.

## What Changes

- Define client-direction VGF1 type `0x03` as an empty terminal receipt in the existing frame grammar.
- Add fixed conformance vectors for valid receipt handling and invalid direction, body, truncation, length, and unknown-type cases.
- Generate consistent Go, Rust, Swift, and Python validation behavior from the Protocol source.

## Capabilities

### New Capabilities

None.

### Modified Capabilities

- `gateway-input-feedback`: Make the already-required terminal receipt executable and cross-language conformant without weakening other VGF1 validation.

## Impact

Protocol frame documentation, conformance fixtures, generators, generated bindings, native validator tools, and immutable Protocol consumers. RC8 remains unchanged and a new immutable Protocol version is required.