fix(cli): default dev bundler to Turbopack (16.2.x panic no longer reproduces) (#5206)

Integrated into release/v3.8.39

.env.example

@@ -111,13 +111,19 @@ PORT=20128
 # Used by: src/app/api/v1/relay/chat/completions/route.ts
 # RELAY_IP_PER_MINUTE=30
 
-# Bundler selection for `npm run dev`. Set to 1 to opt into Turbopack.
-# Default is 0 (webpack) because Turbopack 16.2.x panics on the OmniRoute
-# module graph with "internal error: entered unreachable code: there must be
-# a path to a root" (turbopack-core/module_graph/mod.rs:662). Same bug class
-# the production Docker build worked around in PR #4052. Webpack starts
-# slower but compiles cleanly. Re-enable once upstream Turbopack ships a fix.
-OMNIROUTE_USE_TURBOPACK=0
+# Bundler selection for `npm run dev`. Set to 0 to fall back to webpack.
+# Default is 1 (Turbopack). PR #4092 had forced webpack because earlier
+# Turbopack 16.2.x panicked on the OmniRoute module graph with "internal error:
+# entered unreachable code: there must be a path to a root"
+# (turbopack-core/module_graph/mod.rs:662). That panic no longer reproduces on
+# the pinned Next 16.2.9 — verified across a broad cold-compile sweep (36
+# dashboard routes + open-sse-heavy API routes incl. /api/v1/chat/completions,
+# /api/v1/models, /api/mcp) and repeated HMR rebuilds: zero panics. Turbopack
+# also keeps dev memory far lower on the edit→rebuild loop (HMR rebuild RSS stays
+# ~flat vs webpack's monotonic growth), which mitigates the dev-server OOM on
+# this 60+ route app. The production build still uses webpack (build pipeline is
+# unaffected by this dev-only flag).
+OMNIROUTE_USE_TURBOPACK=1
 
 # Skip the SQLite integrity health check on startup (faster boot on large DBs).
 # Used by: src/lib/db/core.ts, src/lib/db/healthCheck.ts. Set to 1 to skip.