feat(lang): rename schedule keyword from extends to modifies

Changed the schedule composition keyword from "extends" to "modifies"
to better reflect the semantic meaning of schedule inheritance. When
a schedule modifies another, it inherits base blocks and can override
them by name or add new blocks.

This is a breaking change for all existing Storybook files that use
schedule composition. The migration is a simple find-and-replace:
  schedule X extends Y → schedule X modifies Y

Changes include:
- Grammar: Updated tree-sitter grammar and lexer token
- Parser: Updated lalrpop parser and AST field names
- Documentation: Updated all reference docs, tutorials, and specs
- Examples: Updated baker-family example schedules
- Tests: Updated all test cases and corpus files
- Testing: Added type system keywords to prop_tests exclusion list
- Tooling: Added xtask for workspace cleanup
- Version: Bumped to v0.3.1 (skipping v0.3.0)
- Spec: Created SBIR v0.3.1 spec documenting the change

BREAKING CHANGE: The "extends" keyword for schedules has been
replaced with "modifies". Update all schedule declarations.

xtask/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "xtask"
+version = "0.1.0"
+edition = "2021"
+publish = false
+
+[dependencies]
+anyhow = "1.0"

xtask/src/main.rs

@@ -0,0 +1,142 @@
+use std::{
+    env,
+    path::PathBuf,
+    process::Command,
+};
+
+use anyhow::{
+    Context,
+    Result,
+};
+
+fn main() -> Result<()> {
+    let task = env::args().nth(1);
+    match task.as_deref() {
+        | Some("clean") => clean()?,
+        | Some(task) => {
+            eprintln!("Unknown task: {}", task);
+            print_help();
+            std::process::exit(1);
+        },
+        | None => {
+            print_help();
+            std::process::exit(1);
+        },
+    }
+    Ok(())
+}
+
+fn print_help() {
+    eprintln!(
+        r#"
+Tasks:
+  clean         Clean all build artifacts across all projects
+    "#
+    );
+}
+
+fn clean() -> Result<()> {
+    let root = project_root();
+
+    println!("🧹 Cleaning Storybook workspace...\n");
+
+    // Clean Rust projects
+    println!("  Cleaning Rust artifacts...");
+    run_command(&["cargo", "clean"], &root)?;
+
+    // Clean tree-sitter-storybook
+    let tree_sitter_dir = root.join("tree-sitter-storybook");
+    if tree_sitter_dir.exists() {
+        println!("  Cleaning tree-sitter-storybook...");
+
+        // Remove node_modules
+        let node_modules = tree_sitter_dir.join("node_modules");
+        if node_modules.exists() {
+            println!("    Removing node_modules/");
+            std::fs::remove_dir_all(&node_modules)
+                .context("Failed to remove tree-sitter-storybook/node_modules")?;
+        }
+
+        // Remove target directory
+        let target = tree_sitter_dir.join("target");
+        if target.exists() {
+            println!("    Removing target/");
+            std::fs::remove_dir_all(&target)
+                .context("Failed to remove tree-sitter-storybook/target")?;
+        }
+
+        // Remove Cargo.lock
+        let cargo_lock = tree_sitter_dir.join("Cargo.lock");
+        if cargo_lock.exists() {
+            println!("    Removing Cargo.lock");
+            std::fs::remove_file(&cargo_lock)
+                .context("Failed to remove tree-sitter-storybook/Cargo.lock")?;
+        }
+
+        // Remove build artifacts
+        let build_dir = tree_sitter_dir.join("build");
+        if build_dir.exists() {
+            println!("    Removing build/");
+            std::fs::remove_dir_all(&build_dir)
+                .context("Failed to remove tree-sitter-storybook/build")?;
+        }
+    }
+
+    // Clean zed-storybook
+    let zed_dir = root.join("zed-storybook");
+    if zed_dir.exists() {
+        println!("  Cleaning zed-storybook...");
+
+        // Remove grammars directory (build artifact)
+        let grammars = zed_dir.join("grammars");
+        if grammars.exists() {
+            println!("    Removing grammars/");
+            std::fs::remove_dir_all(&grammars)
+                .context("Failed to remove zed-storybook/grammars")?;
+        }
+
+        // Remove extension.wasm
+        let wasm = zed_dir.join("extension.wasm");
+        if wasm.exists() {
+            println!("    Removing extension.wasm");
+            std::fs::remove_file(&wasm).context("Failed to remove zed-storybook/extension.wasm")?;
+        }
+    }
+
+    // Clean mdbook artifacts
+    let docs_dir = root.join("docs");
+    if docs_dir.exists() {
+        println!("  Cleaning mdbook artifacts...");
+
+        let book_dir = docs_dir.join("book");
+        if book_dir.exists() {
+            println!("    Removing docs/book/");
+            std::fs::remove_dir_all(&book_dir).context("Failed to remove docs/book")?;
+        }
+    }
+
+    println!("\n✨ Clean complete!");
+    Ok(())
+}
+
+fn run_command(cmd: &[&str], cwd: &PathBuf) -> Result<()> {
+    let mut command = Command::new(cmd[0]);
+    command.args(&cmd[1..]).current_dir(cwd);
+
+    let status = command
+        .status()
+        .with_context(|| format!("Failed to run command: {:?}", cmd))?;
+
+    if !status.success() {
+        anyhow::bail!("Command failed: {:?}", cmd);
+    }
+
+    Ok(())
+}
+
+fn project_root() -> PathBuf {
+    PathBuf::from(env!("CARGO_MANIFEST_DIR"))
+        .parent()
+        .unwrap()
+        .to_path_buf()
+}