Handle full fixed buffers, fix generator edge cases, add a CMake build (2.1.3)

[dallison]

Summary

Messages built in caller-provided buffers now stay inside the buffer when it runs out of space, and phaser builds with -fno-exceptions -fno-rtti. The generator handles two cases that subspace.proto needs, and a CMake build is added so projects outside Bazel can use phaser.

Full fixed buffers (e7e15f7)

• TryCreateMutable returns InvalidArgument for a buffer smaller than its header and ResourceExhausted when the message doesn't fit. CreateMutable aborts in those cases.
• Setters and repeated fields keep their previous contents. mutable_, add_ and array element accessors return a detached handle whose writes go to per-thread scratch storage.
• AllocationFailed() reports the failure. Serialize then fails, Deserialize returns ResourceExhausted, and CreateReadonly treats the buffer as empty.
• Placeholder string handles ignore assignment instead of dereferencing null.
• Without exceptions, misuse that would throw aborts instead.

Generator fixes (3d5eca6)

• Top-level messages are emitted after the same-file messages their fields (and their nested messages' fields) use, so a message can use one defined later in the .proto file.
• When a message has fields named both <name> and has_<name>, the presence accessor for <name> is _phaser_has_<name>(), so it doesn't collide with the has_<name> getter.

CMake build (3d5eca6)

• CMakeLists.txt builds phaser_runtime and protoc-gen-phaser, and defines phaser_add_library(), which generates and builds a library like the Bazel phaser_library rule, short header included.
• A parent project's Abseil, cpp_toolbelt and protobuf targets are used when present; anything missing is fetched with FetchContent.
• The plugin is built only for the host. Cross-compiles take a host plugin in PHASER_PLUGIN_EXECUTABLE.

Dependencies

Requires cpp_toolbelt 2.1.6 (dallison/cpp_toolbelt asil branch), pinned with a git_override to commit 86b45cd until it is in the Bazel Central Registry.

Testing

• bazelisk test //phaser/...: all 25 tests pass, including:
  • New fixed_buffer_test: builds and parses messages in every buffer size up to 4 KB with -fno-exceptions -fno-rtti, and checks that nothing is written past the end.
  • New generator_edge_cases_test: fails to compile without the generator fixes.
• CMake: builds standalone, and inside Subspace with protobuf 29.5, where the generated library and the Subspace ASIL client tests pass. Also cross-compiles for Android x86_64 with a host plugin.