Vendor minimal capnproto

vendor/capnproto/README.md

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+# Vendored Cap'n Proto
+
+Based on Cap'n Proto 1.4.0, the version previously downloaded by this fork.
+
+Source: https://capnproto.org/capnproto-c++-1.4.0.tar.gz
+
+Archive SHA-256: `fa02378ad522b318916b9ad928d1372fc9abd43dd1f4f0392e50450f5c87828f`
+
+See LICENSE.txt and individual files for upstream licenses.
+
+This is an internal serialization/schema-parser subset for pycapnp, not a full
+Cap'n Proto distribution. Generated schema sources are checked in; no compiler
+executables or network downloads are required to build it.
+
+Removed: RPC/capabilities, promises/async I/O, networking/TLS/HTTP, JSON,
+compression, packed/text/stream/fd message serialization, command-line tools and
+code generators, upstream build systems/examples/tests, compiler export helpers,
+unused KJ encodings/stream classes/clocks, and opt-in crash handlers. Also removed
+are canonicalization, borrowed-memory builders, orphan resizing/concatenation,
+schema doc-comment storage, interface-schema support, Windows code, and the
+optional lite build. Interface declarations are rejected with a parser error.
+
+The retained core includes message validation, flat serialization, dynamic
+struct/list/enum values, runtime schema compilation (including generics), and
+its filesystem, allocation, synchronization, and diagnostic dependencies.
+Generated schema reader/builder metadata is retained; pipeline classes are not.
+This source tree is for the Python binding, not a drop-in C++ SDK or schema
+compiler for openpilot's separate C++ build.
+
+The retained `src/capnp/schema.capnp` omits compiler-request and documentation
+metadata. Its generated C++ was rebuilt with upstream 1.4.0's `capnp` and
+`capnpc-c++`, then unused generated pipeline APIs were removed. Regeneration:
+
+```sh
+capnp compile -I<upstream>/src --src-prefix=vendor/capnproto/src/capnp \
+  -o<capnpc-c++>:<output> vendor/capnproto/src/capnp/schema.capnp
+```
+
+Regeneration must also apply the vendor's removal of Pipeline declarations,
+classes and accessors, and Windows-only includes. The unused `LexedTokens` type
+was removed from the shipped generated lexer files; the runtime parser uses
+`LexedStatements`. Lexer reflection metadata is also omitted; its generated
+static reader/builder layouts remain. Generated wire type tags and active field
+ordinals are kept.