Split Windows builds into per-architecture workflows (#553)

* Split staged Windows builds into reusable workflows

- add a shared `reusable-build.yml` workflow for the 16-stage build chain
- add a `prepare` composite action for checkout, Python setup, workspace init, and stage dependency installation
- replace the monolithic `main.yml` workflow with separate `build-x64`, `build-x86`, and `build-arm` entry workflows
- keep release publishing aligned with the new per-architecture workflow structure

* Checkout sources in each reusable build job

- add `actions/checkout@v6` before `prepare` in all reusable build stages
- fetch submodules recursively so local actions and repository contents are available on every fresh runner
- prevent stage-to-stage workflow runs from failing when later jobs start without a checked out workspace

* Configure git identity before upstream rebase

Author: vpday

.github/actions/prepare/action.yml

@@ -0,0 +1,28 @@
+name: Prepare Build Environment
+description: Checkout sources, set up Python, copy workspace, and install stage dependencies
+inputs:
+  python-version:
+    description: Python version
+    required: false
+    default: '3.12'
+runs:
+  using: composite
+  steps:
+    - name: Checkout
+      uses: actions/checkout@v4
+      with:
+        submodules: recursive
+
+    - name: Set up Python ${{ inputs.python-version }}
+      uses: actions/setup-python@v6
+      with:
+        python-version: ${{ inputs.python-version }}
+
+    - name: Init
+      shell: pwsh
+      run: Copy-Item -Path . -Destination 'C:\ungoogled-chromium-windows' -Recurse
+
+    - name: Setup Stage
+      shell: pwsh
+      run: npm install
+      working-directory: ./.github/actions/stage

.github/workflows/build-arm.yml

@@ -0,0 +1,19 @@
+name: build-arm
+on:
+  workflow_dispatch:
+  push:
+    tags:
+      - '*'
+
+permissions:
+  contents: read
+
+concurrency:
+  group: build-arm-${{ github.ref }}
+  cancel-in-progress: false
+
+jobs:
+  build:
+    uses: ./.github/workflows/reusable-build.yml
+    with:
+      arm: true

.github/workflows/build-x64.yml

@@ -0,0 +1,17 @@
+name: build-x64
+on:
+  workflow_dispatch:
+  push:
+    tags:
+      - '*'
+
+permissions:
+  contents: read
+
+concurrency:
+  group: build-x64-${{ github.ref }}
+  cancel-in-progress: false
+
+jobs:
+  build:
+    uses: ./.github/workflows/reusable-build.yml

.github/workflows/build-x86.yml

@@ -0,0 +1,19 @@
+name: build-x86
+on:
+  workflow_dispatch:
+  push:
+    tags:
+      - '*'
+
+permissions:
+  contents: read
+
+concurrency:
+  group: build-x86-${{ github.ref }}
+  cancel-in-progress: false
+
+jobs:
+  build:
+    uses: ./.github/workflows/reusable-build.yml
+    with:
+      x86: true