feat(hyperhdr): vendor the flatbuffers schemas and generated bindings

The .fbs files come from HyperHDR (see schema/NOTICE); the *_generated.rs
files are flatc output regenerated by scripts/regen-fbs.sh, which refuses to
run unless flatc matches the pinned flatbuffers crate.

schema/NOTICE

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+The *.fbs files in this directory are copied from HyperHDR
+(https://github.com/awawa-dev/HyperHDR, include/flatbuffers/parser/,
+commit 3a76dcd, version 24.0.0-beta.0) and are distributed under the
+following license:
+
+MIT License
+
+Copyright (c) 2020-2026 awawa-dev
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

schema/hyperhdr_reply.fbs

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+// Copied verbatim from HyperHDR (https://github.com/awawa-dev/HyperHDR),
+// include/flatbuffers/parser/hyperhdr_reply.fbs @ 3a76dcd (24.0.0-beta.0).
+// Copyright (c) 2020-2026 awawa-dev. MIT License, see schema/NOTICE.
+
+namespace hyperhdrnet;
+
+table Reply {
+  error:string;
+  video:int = -1;
+  registered:int = -1;
+}
+
+root_type Reply;

schema/hyperhdr_request.fbs

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+// Copied verbatim from HyperHDR (https://github.com/awawa-dev/HyperHDR),
+// include/flatbuffers/parser/hyperhdr_request.fbs @ 3a76dcd (24.0.0-beta.0).
+// Copyright (c) 2020-2026 awawa-dev. MIT License, see schema/NOTICE.
+
+namespace hyperhdrnet;
+
+// A priority value of -1 clears all priorities
+table Register {
+  origin:string (required);
+  priority:int;
+}
+
+table RawImage {
+  data:[ubyte];
+  width:int = -1;
+  height:int = -1;
+}
+
+table NV12Image {
+  data_y:[ubyte];
+  data_uv:[ubyte];
+  width:int;
+  height:int;
+  stride_y:int = 0;
+  stride_uv:int = 0;
+}
+
+union ImageType {RawImage, NV12Image}
+
+table Image {
+  data:ImageType (required);
+  duration:int = -1;
+}
+
+table Clear {
+  priority:int;
+}
+
+table Color {
+  data:int = -1;
+  duration:int = -1;
+}
+
+union Command {Color, Image, Clear, Register}
+
+table Request {
+  command:Command (required);
+}
+
+root_type Request;