feat: add @microsoft/fast-build npm package with WASM CLI (#7358)

## Summary

Wraps the `microsoft-fast-build` Rust crate as a WebAssembly module and exposes it as a new npm package `@microsoft/fast-build` with a `fast build` CLI.

## Changes

### `crates/microsoft-fast-build`
- Added `crate-type = ["cdylib", "rlib"]` to `Cargo.toml` to support WASM compilation
- Added `wasm-bindgen` as a **conditional WASM-only dependency** — native targets remain zero-dependency
- Added `src/wasm.rs` exposing two `#[wasm_bindgen]` functions:
  - `render(entry, state)` — render a template with JSON state
  - `render_with_templates(entry, templates_json, state)` — render with custom element templates passed as a JSON map

### `packages/fast-build` (new)
- `package.json` — `@microsoft/fast-build`, **no runtime npm dependencies**; includes a `build` script to regenerate the WASM via `wasm-pack`
- `bin/fast.js` — CLI supporting `fast build` with:
  - `--templates="<glob>"` — glob pattern(s) for custom element HTML templates; warns if not provided or no files match
  - `--output="output.html"` — output file (default: `output.html`)
  - `--entry="index.html"` — entry HTML template (default: `index.html`)
  - `--state="state.json"` — state JSON file (default: `state.json`)
- `wasm/` — pre-built WASM module and JS bindings (generated by `wasm-pack --target nodejs`)

## Notes
- All file I/O and glob resolution is done in plain Node.js with no external dependencies
- The existing Rust unit test suite continues to pass

Author: janechu

build/package.json

@@ -1,5 +1,5 @@
 {
-  "name": "@microsoft/fast-build",
+  "name": "@microsoft/fast-build-tools",
   "description": "Build utilities for FAST monorepo",
   "version": "0.0.0",
   "private": true,

change/@microsoft-fast-build-1761f158-7e17-42b7-8d82-f180cbe96bc4.json

@@ -0,0 +1,7 @@
+{
+  "type": "none",
+  "comment": "Initial addition of npm package for fast CLI",
+  "packageName": "@microsoft/fast-build",
+  "email": "7559015+janechu@users.noreply.github.com",
+  "dependentChangeType": "none"
+}

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")),
+    }
+}