feat(nextjs): add disableWatch option to skip extra env-file watching

[mhornbacher]

Summary

• Adds disableWatch to varlockNextConfigPlugin (sets process.env.__VARLOCK_NEXT_DISABLE_WATCH)
• Next forwards that delta into @next/env updateInitialEnv, which tears down extra-file watchers
• Watched paths are shared across duplicate CJS copies of next-env-compat via globalThis + process.env.__VARLOCK_NEXT_WATCHED_EXTRA_FILES (loadEnvConfig and updateInitialEnv can hit different copies)
• Unit tests cover same-instance and cross-instance teardown

Test plan

• Rebuild/relink @varlock/nextjs-integration
• DEBUG_VARLOCK_NEXT_INTEGRATION=1 with disableWatch: true
• Expect next-env-compat module init <id> (possibly two ids), then after plugin: updateInitialEnv with { instance, newEnv } and disableWatch: unwatchAllExtraFiles { instance, count, paths }
• FIFO .env watch loop stops
• bun run --filter @varlock/nextjs-integration test:ci

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commit: 858e58c

[theoephraim]

I think I solved this with a different tactic - we can just detect the FIFO and not watch it at all.
This still leaves watching/reloads intact when you edit other files.