feat: add @microsoft/fast-build npm package with WASM CLI

- Add wasm-bindgen as a conditional WASM-target dependency to the
  microsoft-fast-build Rust crate (zero-dependency for native targets)
- Add src/wasm.rs exposing render() and render_with_templates() via
  wasm-bindgen for use in Node.js
- Build the crate to WASM (NodeJS target) with wasm-pack
- Create packages/fast-build/ with:
  - package.json (@microsoft/fast-build, no runtime npm dependencies)
  - bin/fast.js: CLI supporting `fast build` with arguments:
      --templates="<glob>"  custom element HTML templates (warns if missing)
      --output="output.html" (default: output.html)
      --entry="index.html"  (default: index.html)
      --state="state.json"  (default: state.json)
  - wasm/: compiled WASM module and JS bindings

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: janechu

crates/microsoft-fast-build/Cargo.lock

@@ -13,3 +13,7 @@ categories = ["template-engine", "web-programming"]
 [lib]
 name = "microsoft_fast_build"
 path = "src/lib.rs"
+crate-type = ["cdylib", "rlib"]
+
+[target.'cfg(target_arch = "wasm32")'.dependencies]
+wasm-bindgen = "0.2"

crates/microsoft-fast-build/Cargo.toml

@@ -9,6 +9,8 @@ mod node;
 mod renderer;
 mod error;
 mod locator;
+#[cfg(target_arch = "wasm32")]
+mod wasm;
 
 pub use json::{JsonValue, JsonError};
 pub use error::RenderError;

crates/microsoft-fast-build/src/lib.rs

@@ -0,0 +1,42 @@
+use wasm_bindgen::prelude::*;
+use std::collections::HashMap;
+use crate::{json, Locator, render_template, render_template_with_locator};
+
+/// Render a FAST HTML template with a JSON state string.
+/// Returns the rendered HTML or throws a JavaScript error.
+#[wasm_bindgen]
+pub fn render(entry: &str, state: &str) -> Result<String, JsValue> {
+    render_template(entry, state).map_err(|e| JsValue::from_str(&e.to_string()))
+}
+
+/// Render a FAST HTML template with custom element templates and a JSON state string.
+/// `templates_json` is a JSON object mapping element names to their HTML template strings,
+/// e.g. `{"my-button": "<template>...</template>"}`.
+/// Returns the rendered HTML or throws a JavaScript error.
+#[wasm_bindgen]
+pub fn render_with_templates(entry: &str, templates_json: &str, state: &str) -> Result<String, JsValue> {
+    let templates = parse_templates_map(templates_json)?;
+    let locator = Locator::from_templates(templates);
+    render_template_with_locator(entry, state, &locator)
+        .map_err(|e| JsValue::from_str(&e.to_string()))
+}
+
+fn parse_templates_map(templates_json: &str) -> Result<HashMap<String, String>, JsValue> {
+    let parsed = json::parse(templates_json)
+        .map_err(|e| JsValue::from_str(&format!("Failed to parse templates JSON: {}", e.message)))?;
+    match parsed {
+        json::JsonValue::Object(obj) => {
+            let mut map = HashMap::new();
+            for (k, v) in obj {
+                match v {
+                    json::JsonValue::String(s) => { map.insert(k, s); }
+                    _ => return Err(JsValue::from_str(&format!(
+                        "Template value for '{}' must be a string", k
+                    ))),
+                }
+            }
+            Ok(map)
+        }
+        _ => Err(JsValue::from_str("Templates must be a JSON object")),
+    }
+}