build(bun): allow Turbopack bundler flag on Bun 1.4+ with configurable Webpack fallback (#11471)

Validated in a combined 4-PR batch worktree off release/v3.8.51 tip (last of the Bun-native cluster; conflicted against the already-merged #11482's Dockerfile.bun hunk in the shared worktree — resolved by taking this PR's configurable ARG/ENV shape, which is exactly what it's designed to replace, and pushed the same resolution to this branch).
- Focused test: resolve-next-build-bundler-flag.test.mjs — 3/3 pass, part of batch's 5/5 node:test run
- typecheck:core, file-size, changelog-integrity, complexity, cognitive-complexity — all OK
- Full-repo lint: 228 pre-existing dashboard react-hooks/* findings, unrelated to this diff

Thanks for validating with both node --test and bun test — good practice given the dual-runtime surface this touches.

scripts/build/build-next-isolated.mjs

@@ -131,9 +131,9 @@ function runNextBuild() {
 }
 
 export function resolveNextBuildBundlerFlag(baseEnv = process.env) {
-  // Turbopack is the default on Node.js; on Bun or when explicitly disabled (=0),
-  // use Webpack (--webpack) to avoid Turbopack V8 internal worker API mismatches.
-  if (process.versions.bun || baseEnv.OMNIROUTE_USE_TURBOPACK === "0") {
+  // Turbopack is the default (on Node.js and Bun 1.4+).
+  // When explicitly disabled (OMNIROUTE_USE_TURBOPACK=0), use Webpack (--webpack) as fallback.
+  if (baseEnv.OMNIROUTE_USE_TURBOPACK === "0") {
     return "--webpack";
   }
   return "--turbopack";