FOLLOW-UP: Add serve success-path smoke test (from PR #195)

[adnaan]

Context

This follow-up task was identified during the review of PR #195.

Source PR: #195
PR Title: test: add CLI testscript suite with 25 black-box tests
Suggested by: @claude[bot]

Task Description

Add a testscript that validates the happy path for tinkerdown serve. Currently serve_errors.txtar only tests error cases (nonexistent directory, invalid port). A smoke test that starts the server and verifies it binds to a port would be valuable.

Challenge: The serve command blocks, so this would require background execution and a timeout pattern within testscript.

Original Comment

serve success path — similarly, only error cases are tested. A smoke test that starts serve and checks it binds (with a timeout/background exec) would be valuable, though this is admittedly harder with testscript.

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