🐳 Add devcontainer with pinned Rust toolchain and build dependencies

.devcontainer/Dockerfile

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+FROM mcr.microsoft.com/devcontainers/base:ubuntu
+
+# Install system dependencies
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    pandoc \
+    curl \
+    git \
+    build-essential \
+    pkg-config \
+    libssl-dev \
+    ca-certificates \
+    && rm -rf /var/lib/apt/lists/*
+
+# Install Rust via rustup (stable toolchain)
+ENV RUSTUP_HOME=/usr/local/rustup \
+    CARGO_HOME=/usr/local/cargo \
+    PATH=/usr/local/cargo/bin:$PATH
+
+RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | \
+    sh -s -- -y --no-modify-path --default-toolchain stable && \
+    chmod -R a+w $RUSTUP_HOME $CARGO_HOME
+
+# Install additional Cargo tooling
+RUN cargo install cross --locked && \
+    cargo install cargo-watch --locked
+
+# Install Taskfile (go-task) – pinned to v3.43.3, downloaded directly from GitHub releases
+ARG TASK_VERSION=v3.43.3
+RUN curl --proto '=https' --tlsv1.2 -fsSL \
+    "https://github.com/go-task/task/releases/download/${TASK_VERSION}/task_linux_amd64.tar.gz" \
+    -o /tmp/task.tar.gz && \
+    tar -xzf /tmp/task.tar.gz -C /usr/local/bin task && \
+    rm /tmp/task.tar.gz && \
+    chmod +x /usr/local/bin/task
+
+# Ensure cargo bin is on PATH for all users / shells
+RUN echo 'export PATH="/usr/local/cargo/bin:$PATH"' >> /etc/bash.bashrc
+
+WORKDIR /workspaces/renderflow
+
+SHELL ["/bin/bash", "-c"]

.devcontainer/devcontainer.json

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+{
+  "name": "Renderflow Dev Container",
+  "build": {
+    "dockerfile": "Dockerfile",
+    "context": ".."
+  },
+  "workspaceFolder": "/workspaces/renderflow",
+  "workspaceMount": "source=${localWorkspaceFolder},target=/workspaces/renderflow,type=bind,consistency=cached",
+  "settings": {
+    "terminal.integrated.defaultProfile.linux": "bash"
+  },
+  "extensions": [
+    "rust-lang.rust-analyzer",
+    "tamasfe.even-better-toml",
+    "ms-azuretools.vscode-docker"
+  ],
+  "remoteEnv": {
+    "PATH": "/usr/local/cargo/bin:${containerEnv:PATH}"
+  },
+  "postCreateCommand": "cargo fetch",
+  "remoteUser": "vscode"
+}