ci : reduce builds in build-xcframework.sh (#27252)

* ci : parallelize platform builds in build-xcframework.sh

The ios-xcode release job builds 7 platform/simulator configurations
sequentially, each with -j $(nproc). Run them with at most 3 concurrent
builds (the release runner has 3 cores), splitting the cores between the
builds (-j 1 each on the runner), so total CPU pressure is unchanged while
the build phase runs about 2.3x faster.

- convert the 7 build blocks into functions (flags unchanged)
- add a run_builds_parallel pool: 3-slot sliding window, per-build logs,
  dumps the failing log and aborts on error (background job failures do
  not trigger set -e)
- queue the 2-arch builds first so the slower builds occupy the slots early

Assisted-by: pi:llama.cpp/Qwen3.8-27B

* pi : add guideline for comments

* cont : disable 5/7 builds

* ci : make build-xcframework.sh builds configurable via CLI args

The script now takes an optional list of builds to run
(ios-sim ios-device macos visionos visionos-sim tvos-sim tvos-device);
with no arguments it builds all of them, as before. The per-build
lists for the build pool, framework setup, static library combining
and xcframework creation are now driven by a single build_spec
lookup instead of four hardcoded (partially commented-out) lists.

release.yml builds only macos and ios-device to cut the build time.

Assisted-by: pi:llama.cpp/Qwen3.8-27B

.github/workflows/release.yml

@@ -1439,7 +1439,9 @@ jobs:
       - name: xcodebuild for swift package
         id: xcodebuild
         run: |
-          ./build-xcframework.sh
+          # note: only macos and ios-device due to long build time
+          # ref: https://github.com/ggml-org/llama.cpp/pull/27252
+          ./build-xcframework.sh macos ios-device
 
       - name: Build Xcode project
         run: xcodebuild -project examples/llama.swiftui/llama.swiftui.xcodeproj -scheme llama.swiftui -sdk iphoneos CODE_SIGNING_REQUIRED=NO CODE_SIGN_IDENTITY= -destination 'generic/platform=iOS' FRAMEWORK_FOLDER_PATH=./build-ios build