Stop stats after missing transcript

[w287346141]

Summary

• return immediately after reporting a missing transcript path in the stats command
• add a regression test that mocks process.exit and verifies the command does not continue into readFileSync

Verification

• npm.cmd test -- --run tests/stats-command.test.ts
• npm.cmd run lint -- src/cli/commands/stats.ts tests/stats-command.test.ts
• npm.cmd run typecheck
• npm run verify (pre-push hook)