fix(build): exit watch mode cleanly on SIGINT

Both `scripts/build/main.mts` and `scripts/build/js.mts` had the same
pattern in their watch-mode SIGINT handlers: set `process.exitCode = 0`,
then `throw new Error('Watch mode interrupted')`. The throw happens
inside an `async () => {}` registered with `process.on('SIGINT', ...)`,
so it surfaces as an unhandled promise rejection — which Node exits
non-zero on, negating the `exitCode = 0` — and even when the rejection
is caught by the surrounding try/catch of `watchBuild`, the user sees
`Watch mode failed: Watch mode interrupted` instead of a clean exit.

Replaces the throw with `ctx.dispose().finally(() => process.exit(0))`,
which tears down the esbuild context then exits with status 0 whether
dispose succeeds or fails. Dev-loop Ctrl-C now closes without a stack
trace.

Author: jdalton

scripts/build/js.mts

@@ -98,18 +98,19 @@ async function watchJS(): Promise<number> {
 
     await ctx.watch()
 
-    // Keep process alive
-    process.on('SIGINT', async () => {
+    // On Ctrl-C, tear down the esbuild context and exit cleanly. Earlier
+    // this handler threw inside an async callback, which surfaced as an
+    // unhandled rejection and the outer try/catch rewrote the clean exit
+    // into "Watch mode failed: Watch mode interrupted".
+    process.on('SIGINT', () => {
       if (!isQuiet) {
         logger.log('\nStopping watch mode...')
       }
-      await ctx.dispose()
-      process.exitCode = 0
-      throw new Error('Watch mode interrupted')
+      ctx.dispose().finally(() => process.exit(0))
     })
 
-    // Wait indefinitely
-    await new Promise(() => {})
+    // Wait indefinitely — SIGINT is the only exit path.
+    await new Promise<never>(() => {})
   } catch (error) {
     if (!isQuiet) {
       logger.error('Watch mode failed')

scripts/build/main.mts

@@ -252,15 +252,18 @@ async function watchBuild(
     // Enable watch mode
     await ctx.watch()
 
-    // Keep the process alive
-    process.on('SIGINT', async () => {
-      await ctx.dispose()
-      process.exitCode = 0
-      throw new Error('Watch mode interrupted')
+    // On Ctrl-C, tear down the esbuild context and exit cleanly. Earlier
+    // this handler threw inside an async callback, which surfaced as an
+    // unhandled rejection and the surrounding try/catch rewrote the clean
+    // exit into "Watch mode failed: Watch mode interrupted".
+    process.on('SIGINT', () => {
+      ctx.dispose().finally(() => process.exit(0))
     })
 
-    // Wait indefinitely
-    await new Promise(() => {})
+    // Wait indefinitely — SIGINT is the only exit path.
+    await new Promise<never>(() => {})
+    // Unreachable; satisfies Promise<number> return type.
+    return 0
   } catch (error) {
     if (!quiet) {
       logger.error('Watch mode failed:', error)