Add Dev Container for Codespaces + CI configuration

.devcontainer/Dockerfile

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+FROM rust:1-bookworm
+
+# Install build and OpenCL headers (compile-time), BLAS/LAPACK, git, curl
+RUN apt-get update \
+    && apt-get install -y --no-install-recommends \
+        build-essential \
+        pkg-config \
+        git \
+        curl \
+        ocl-icd-opencl-dev \
+        clinfo \
+        libblas-dev \
+        liblapack-dev \
+    && rm -rf /var/lib/apt/lists/*
+
+# Set a default workdir used by Dev Containers / Codespaces
+WORKDIR /workspaces/ha-ndarray
+

.devcontainer/devcontainer.json

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+{
+  "name": "ha-ndarray",
+  "build": {
+    "dockerfile": "Dockerfile"
+  },
+  "remoteUser": "root",
+  "containerEnv": {
+    "RUST_BACKTRACE": "1"
+  },
+  "postCreateCommand": "rustup component add clippy rustfmt && cargo fetch",
+  "customizations": {
+    "vscode": {
+      "extensions": [
+        "rust-lang.rust-analyzer",
+        "tamasfe.even-better-toml"
+      ]
+    }
+  }
+}

.github/workflows/devcontainer.yml

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+name: Dev Container
+
+on:
+  pull_request:
+  push:
+    branches: [ devcontainer-codespaces ]
+
+jobs:
+  build-devcontainer:
+    name: Build .devcontainer/Dockerfile (${{ matrix.platform }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        platform: [ linux/amd64, linux/arm64 ]
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up QEMU (emulation for non-native arch)
+        uses: docker/setup-qemu-action@v3
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Build (cache-enabled)
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          file: .devcontainer/Dockerfile
+          platforms: ${{ matrix.platform }}
+          push: false
+          load: true
+          cache-from: type=gha
+          cache-to: type=gha,mode=max

.github/workflows/notify-devcontainer.yml

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+name: Notify Devcontainer Status
+
+on:
+  workflow_run:
+    workflows: ["Dev Container"]
+    types: [completed]
+
+jobs:
+  notify:
+    runs-on: ubuntu-latest
+    permissions:
+      issues: write
+      pull-requests: write
+      actions: read
+    steps:
+      - name: Comment PR with results
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const run = context.payload.workflow_run;
+            const { data } = await github.rest.actions.listJobsForWorkflowRun({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              run_id: run.id,
+              per_page: 100,
+            });
+
+            const lines = (data.jobs || []).map(j => `- ${j.name}: ${j.conclusion}`);
+            const body = [
+              `Dev Container workflow completed: ${run.conclusion}`,
+              ``,
+              `Jobs:`,
+              ...lines,
+              ``,
+              `Run: ${run.html_url}`,
+            ].join('\n');
+
+            const prs = run.pull_requests || [];
+            if (prs.length === 0) {
+              core.info('No associated PRs to notify.');
+              return;
+            }
+
+            for (const pr of prs) {
+              await github.rest.issues.createComment({
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                issue_number: pr.number,
+                body,
+              });
+            }
+