refactor(ipc-codegen/zig): collapse codegen onto ipc-runtime/zig

[charlielye]

Summary

Stacked on #23498 (cl/ipc-runtime-ts → cl/ipc-bbapi-server-signals → cl/ipc-codegen-migrate-bb-wsdb).

Final language in the cross-language collapse — Zig codegen now uses the shared `ipc-runtime/zig` transport instead of per-service UDS templates.

Changes

Templates deleted (all reimplemented length-prefix UDS framing that ipc-runtime/zig already exposes):

• `ipc-codegen/templates/zig/uds_backend.zig`
• `ipc-codegen/templates/zig/ipc_server.zig`
• `ipc-codegen/templates/zig/ipc_client.zig` (was already unreferenced)
• `ipc-codegen/templates/zig/ffi_client.zig` (was already unreferenced)
• `backend.zig` stays for FFI consumers.

Codegen (`zig_codegen.ts`):

• `generateServer` no longer emits a `serve(path)` wrapper that pulled in the deleted template. Now emits `dispatch` + `DispatchResult` + handler stubs only; the consumer wires the transport (recommended: `ipc_runtime.Server.fromPath/listen/run`).

Runtime (`ipc-runtime/zig/src/main.zig`):

• Added `Client.destroy()` as an alias for `deinit()` so the runtime's `Client` satisfies the codegen's `Backend` contract (expects `destroy(self: *T) void`) and slots directly into the generated `Client(BackendType)`.

Echo example:

• `echo_client.zig` imports `ipc_runtime` and uses `ipc_runtime.Client` as the Backend.
• `echo_server.zig` uses `ipc_runtime.Server.fromPath/listen/run` with manual msgpack decode/encode (the codegen's `dispatch()` is invoked from the byte handler).
• `build.zig` + `build.zig.zon` add the ipc_runtime path dep.

Test plan

• All zig templates referenced for UDS are gone: `ls ipc-codegen/templates/zig/` → `backend.zig ffi_backend.zig`
• `ipc-codegen ./bootstrap.sh test` → 18/18 cross-language tests green
• `zig build` clean in the echo example