fix(bcode-browser): detect compiled mode on Windows; pre-flight harnessDir

The bunfs root is /$bunfs/ on POSIX but B:\~BUN\ on Windows (native
separators), so `__dirname.startsWith("B:/~BUN/")` was always false in
Windows compiled binaries. Result: isCompiled=false, resolveHarnessDir
returned the dev path which doesn't exist on installs, the harness was
never extracted, and every spawn failed with cwd=ENOENT. Bun's spawn
attributes that ENOENT to path="uv", so isUvMissing matched and the
agent reported a misleading 'uv not on PATH' error -- even though uv
was installed and on PATH.

Two fixes:
1. harness.ts: normalize __dirname to forward slashes before the bunfs
   prefix check, so the same comparison works on Windows and POSIX.
2. browser-execute.ts: fs.access(harnessDir) before spawn. If the
   directory is missing, fail with a clear message instead of falling
   into the spawn-ENOENT path that gets misclassified as uv-missing.

Reproduced on Windows 11 with v0.0.5 binary (and a locally-built one).
Pre-fix: ~/.cache/bcode/harness was never created; agent reported uv
missing despite uv being installed and on PATH.
Post-fix: harness extracts on first run, agent attaches to Chrome,
end-to-end browser_execute works.

Author: Alezander9

packages/bcode-browser/src/browser-execute.ts

@@ -92,6 +92,13 @@ export const make = Effect.fn("BrowserExecute.make")(function* () {
   const execute = (args: Parameters, ctx: ExecuteContext) =>
     Effect.gen(function* () {
       const harnessDir = yield* Effect.promise(() => resolveHarnessDir())
+      // Pre-flight check on harnessDir: spawn ENOENT on a missing cwd surfaces
+      // with `path: "uv"` on Bun/Windows, which is indistinguishable from a
+      // truly-missing uv. Catch it here so the user gets the real cause
+      // instead of a misleading "uv not on PATH" hint.
+      if (!(yield* Effect.promise(() => fs.access(harnessDir).then(() => true, () => false)))) {
+        return yield* Effect.fail(new Error(`harness directory not found at ${harnessDir} — bcode build is broken; please reinstall`))
+      }
       yield* Effect.promise(() => fs.mkdir(ctx.bhTmpDir, { recursive: true }))
       const uv = yield* locate
       const proc = ChildProcess.make(

packages/bcode-browser/src/harness.ts

@@ -37,7 +37,15 @@ import path from "path"
 import { fileURLToPath } from "url"
 
 const __dirname = path.dirname(fileURLToPath(import.meta.url))
-const isCompiled = __dirname.startsWith("/$bunfs/") || __dirname.startsWith("B:/~BUN/")
+// Bun's bunfs root is `/$bunfs/` on POSIX and `B:\~BUN\` on Windows (native
+// separators). Normalize before comparing so the compiled-mode check works on
+// both platforms — without this, the Windows compiled binary falls through to
+// DEV_HARNESS_DIR (which doesn't exist on the user's machine) and every
+// subsequent spawn fails with a misleading uv-missing error.
+const isCompiled = (() => {
+  const d = __dirname.replaceAll("\\", "/")
+  return d.startsWith("/$bunfs/") || d.startsWith("B:/~BUN/")
+})()
 const DEV_HARNESS_DIR = path.resolve(__dirname, "..", "harness")
 const cachedHarnessDir = path.join(os.homedir(), ".cache", "bcode", "harness")