MCP server: modelcraft-cli mcp (headless or bridged), read/write roots, end-to-end analysis over MCP (#48)

* io: read and write roots that confine an automation client's file access

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* engine: every file a command reads or writes is checked against the session's roots

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* engine: headless pictures of a session (model, geometry, deformed shape and contours)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* mcp: MCP server over stdio with headless and connected backends, roots and caps

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* cli: modelcraft-cli mcp [--connect ADDR | --headless] with automation read and write roots

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* mcp: end-to-end analysis of a corpus part over MCP (open, tet mesh, define, solve, probe, render)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

* docs: MCP server (docs/mcp.md), Claude Code setup, roots; architecture, README, roadmap, attribution

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

---------

Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/mcp/src/lib.rs

@@ -0,0 +1,56 @@
+//! ModelCraft's MCP server (docs/mcp.md).
+//!
+//! JSON-RPC 2.0 over stdio, one message per line, implementing the Model Context Protocol's
+//! lifecycle, tools and resources. Every tool runs engine commands (the same ones the UI,
+//! scripts, the CLI and the control channel run) and never opens a dialog. A tool call is served
+//! by a [`Backend`]: an in-process headless session ([`Headless`], pictures drawn with the CPU
+//! rasterizer) or a running app reached through its control channel ([`Remote`],
+//! docs/control-protocol.md), so a person can watch the agent work.
+//!
+//! The server treats its input as hostile: a message line is capped at [`MAX_LINE`] bytes and
+//! answered with an error past it, a JSON-RPC batch at [`MAX_BATCH_MESSAGES`] messages, a
+//! `batch` tool call at [`tools::MAX_STEPS`] commands, and tool arguments are checked against
+//! the tool's schema before anything runs. Read and write roots ([`modelcraft_io::roots`])
+//! confine every file a tool touches.
+//!
+//! Adapted from SolidCraft's `crates/mcp` (MIT OR Apache-2.0), see ATTRIBUTION.md.
+#![deny(clippy::unwrap_used, clippy::expect_used, clippy::panic, clippy::unimplemented, clippy::todo, clippy::unreachable)]
+#![forbid(unsafe_code)]
+
+mod backend;
+mod server;
+pub mod tools;
+
+pub use backend::{Backend, Headless, Picture, PictureRequest, Remote};
+pub use server::{MAX_BATCH_MESSAGES, MAX_LINE, PROTOCOL_VERSION, SUPPORTED_VERSIONS, Server};
+
+use modelcraft_io::roots::Roots;
+
+/// How the server is started (`modelcraft-cli mcp`).
+#[derive(Debug, Clone, Default)]
+pub struct Options {
+    /// The control channel of a running app (`127.0.0.1:7070`); `None` runs headless.
+    pub connect: Option<String>,
+    /// Folders file access is confined to; unconfined by default.
+    pub roots: Roots,
+}
+
+/// The backend `options` ask for, and a note for stderr when the server falls back to headless
+/// because the app cannot be reached. Confined roots with `connect` are refused: the app would
+/// read and write files the roots do not cover.
+pub fn backend(options: Options) -> Result<(Box<dyn Backend>, Option<String>), String> {
+    let headless = |roots: Roots| -> Box<dyn Backend> { Box::new(Headless::new(roots)) };
+    match options.connect {
+        Some(addr) if options.roots.is_confined() => Err(format!(
+            "--connect {addr} cannot be combined with --automation-read-root or --automation-write-root: the app is not confined by them (they imply --headless)"
+        )),
+        Some(addr) => match Remote::connect(&addr) {
+            Ok(r) => Ok((Box::new(r), None)),
+            Err(e) => Ok((headless(options.roots), Some(format!("ModelCraft at {addr} cannot be reached ({e}); running headless")))),
+        },
+        None => Ok((headless(options.roots), None)),
+    }
+}
+
+#[cfg(test)]
+mod tests;