build_sdk: pin SDK v2.8.6 (recorder API) and stop double-nesting the build dir

The recorder uses ob::Error::getStatus(), present in v2.8.6 (the Nano) but not
v2.4.3 (the version pre-placed on the Orin), so pin the version the recorder is
written against. Also configure from $root/build with 'cmake ..' — the SDK's
cmake makes its own linux_<arch> output subdir, so creating one first put the
libs at build/linux_aarch64/linux_arm64/lib where find_build could not see them.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/build_sdk.sh

@@ -26,7 +26,10 @@ repo=$(cd "$here/.." && pwd)
 eval "$("$here/detect_platform.sh")"
 
 SDK_REPO="${OB_SDK_REPO:-https://github.com/orbbec/OrbbecSDK_v2.git}"
-SDK_REF="${OB_SDK_REF:-main}"
+# Pinned to the version the recorder source is written against: v2.4.3 (which came
+# pre-placed on the Orin) lacks ob::Error::getStatus() that the recorder uses. A
+# caller can override for a platform that needs a different tag.
+SDK_REF="${OB_SDK_REF:-v2.8.6}"
 
 find_build() {
     local root="$1" b
@@ -100,11 +103,14 @@ inject_recorder "$root"
 cmake_k4a=()
 [[ -n "${K4A_WRAPPER_DIR:-}" ]] && cmake_k4a=(-DK4A_WRAPPER_DIR="$K4A_WRAPPER_DIR")
 
-build="$root/build/linux_$(uname -m)"
+# Build in $root/build and configure from there (cmake ..): the SDK's own
-mkdir -p "$build"
+# linux_config.cmake places its outputs in a build/linux_<arch> subdir itself.
+# Creating that subdir and configuring from inside it double-nests the outputs
+# (build/linux_aarch64/linux_arm64/lib), where find_build cannot see them.
+mkdir -p "$root/build"
 (
-    cd "$build"
+    cd "$root/build"
-    cmake ../.. -DCMAKE_BUILD_TYPE=Release -DOB_BUILD_EXAMPLES=ON "${cmake_k4a[@]}" >&2
+    cmake .. -DCMAKE_BUILD_TYPE=Release -DOB_BUILD_EXAMPLES=ON "${cmake_k4a[@]}" >&2
     make -j"$(nproc)" >&2
 )