fix: honor underscore-unused convention and make durability fsyncs cross-platform

- eslint: respect the repo's existing _-prefix convention for unused vars
- phase-one-blockers: derive projectRoot via import.meta.dirname so Windows
  checkouts stop producing C:\C:\... paths
- backup/release-evidence/cutover-orchestrator: skip POSIX-only directory
  fsync on win32 and fsync read-only handles through a writable handle

test/phase-one-blockers.test.js

@@ -14,7 +14,7 @@ import { RequestCoordinator } from '../public/state/request-coordinator.js'
 import { createApiClient } from '../public/infrastructure/api-client.js'
 import { createViewRouter } from '../public/router/view-router.js'
 
-const projectRoot = path.resolve(path.dirname(new URL(import.meta.url).pathname), '..')
+const projectRoot = path.resolve(import.meta.dirname, '..')
 
 async function files(instances = []) {
   const dir = await mkdtemp(path.join(tmpdir(), 'msa-blockers-'))