Linux BUILD_FILAMENT_FROM_SOURCE lists archives under lib instead of lib/x86_64

### Checklist

- [x] Clean upstream checkout at the exact reported revision.
- [x] Built `pip-package` directly without prebuilding `ext_filament`.
- [x] Preserved the original Ninja error and compared only the archive-directory correction.

### Environment

```text
Ubuntu 26.04.1 LTS, x86_64
Linux 7.0.0-31-generic
Python 3.12.13
CMake 4.2.3; Ninja 1.13.2
GCC 15.2.0; Clang 21.1.8
Open3D 1a9eb990f9a20936c30c428568c602bdef760744
Filament v1.54.0, tag commit c1a3450d9c0a64c329a17bff4f3a7f68a2474214
Clean checkout status before configure: empty
```

### Exact reproduction

```bash
cmake -S . -B build-byproducts -G Ninja \
-DCMAKE_BUILD_TYPE=Release \
-DBUILD_GUI=ON -DBUILD_FILAMENT_FROM_SOURCE=ON \
-DBUILD_CUDA_MODULE=OFF -DBUILD_WEBRTC=OFF \
-DPython3_EXECUTABLE=/path/to/python

set -o pipefail
cmake --build build-byproducts --target pip-package --parallel 24 --verbose 2>&1 \
| tee build-original.log
status=${PIPESTATUS[0]}
printf '\nExit status: %s\n' "$status" | tee -a build-original.log
exit "$status"
```

I did not prebuild `ext_filament`. The generated graph fails immediately and deterministically:

```text
Change Dir: '/tmp/open3d-audit-7555-build'
Run Build Command(s): /usr/bin/ninja -v pip-package
ninja: error: 'filament-binaries/lib/x86_64/libfilameshio.a', needed by 'lib/Release/libOpen3D.so.0.19.0', missing and no known rule to make it

Exit status: 1
```

### Same-build graph evidence

At this revision, `3rdparty/filament/filament_build.cmake` leaves Linux `lib_dir` as `lib`, so the Ninja custom command declares these byproducts:

```text
filament-binaries/lib/libfilameshio.a
filament-binaries/lib/libfilament.a
...
filament-binaries/lib/libvkshaders.a
```

The imported Filament targets in `3rdparty/find_dependencies.cmake`, and Filament's actual install layout, instead use:

```text
filament-binaries/lib/x86_64/libfilameshio.a
filament-binaries/lib/x86_64/libfilament.a
...
filament-binaries/lib/x86_64/libvkshaders.a
```

Ninja therefore has no rule for the imported archive path even though the external project is the intended producer.

### Isolated archive-directory correction

The control changed only Linux archive-directory selection:

```cmake
elseif(UNIX)
if(CMAKE_SYSTEM_PROCESSOR MATCHES "^(aarch64|arm64)$")
set(lib_dir lib/aarch64)
else()
set(lib_dir lib/x86_64)
endif()
endif()
```

After regeneration, both the `ext_filament` phony edge and external-project build edge declare all 15 archives under `filament-binaries/lib/x86_64/`. The archives were then present in that directory, including:

```text
b8b23a5152e0ae2c70feeacdfdbcc35795fdca608090647e25db46708f7640d6 lib/x86_64/libfilameshio.a
938d9cfbd4ad8ef5cab54a5485adb01e00141ab777a63c23eeb1f3801b65466e lib/x86_64/libfilament.a
8900084f7901f938be163b48d1dbb44507d1e2fd4f88eeb57ba5ef981e378cdf lib/x86_64/libvkshaders.a
```

The direct `pip-package` build then completed with exit status 0. Produced wheel:

```text
0f02385e6f0bcc90f2ae54bfd02cc9b942a9c9ffa35918f254494b59383a9167 open3d_cpu-0.19.0+1a9eb99-cp312-cp312-manylinux_2_43_x86_64.whl
```

### Separately recorded host prerequisites

These were required only after the graph defect was corrected and are not part of the candidate fix:

- Filament v1.54.0 with Clang 21 needs `` in `libs/utils/include/utils/memalign.h` plus compatibility warning flags.
- This Ubuntu/GCC 15 host needs a temporary `-include cstdint` workaround for unrelated Assimp/tinyusdz missing includes.
- `WITH_STUBGEN=OFF` was used because `pybind11_stubgen` is not installed.

The original missing-rule failure occurs before any of those compilation prerequisites matter.

### Result

Confirmed on Linux x86_64. The declared-byproduct directory is `lib/`, while both Open3D imports and Filament installation use `lib/x86_64/`. Selecting the architecture-specific directory for the external project's `BUILD_BYPRODUCTS` fixes the Ninja graph and the full target succeeds. This report does not claim an untested architecture.

Start in 3rdparty/filament/filament_build.cmake and compare its Linux lib_dir and BUILD_BYPRODUCTS entries with the imported archive paths in 3rdparty/find_dependencies.cmake. Reproduce the reported cmake configuration and pip-package Ninja build on Linux x86_64, then verify that the generated graph finds all 15 archives and the target completes successfully. The report does not establish behavior on other architectures.

Written by the indexing model from the issue text.