chat.test.ts proves a timed mute survives a reload and still ends on time - test only #415

Replaces #256. Timed mutes are on main in 81fbc14: Chat.mute(id, name, until) in Software/so101/src/chat.ts, with an end time or null for good. #256 asked for a new muteUntil instead, and builders kept leaving it because there was nothing to add. The one criterion still unproven is the reload of a timed mute: the existing test reloads only the permanent one.

Done:

• [ ] A test in Software/so101/src/chat.test.ts, with an injected clock: a 10-minute mute is set, the Chat is rebuilt from the same file at 5 minutes and still refuses that person's line, and at 10:01 it accepts the line and the person is no longer in muted.
• [ ] If the test fails on main, fix chat.ts so it passes, and say so in the pull. Otherwise only the test file changes.
• [ ] forge.json untouched. bun x tsc --noEmit and bun --cwd Software/so101 test pass.