feat: credit the triggering chatter on the timewarp overlay

[dmerrick]

What

When a viewer runs !timewarp or guesses the state correctly with !guess, their username now appears as a credit line under the TIMEWARP wordmark on the full-screen warp overlay (e.g. @viewer1).

How it flows

The username travels the existing onscreens NATS command surface end-to-end:

1. guessCmd / timewarpCmd → a.timewarp(ctx, user.Username)
2. App.Onscreens.ShowTimewarp(ctx, username) → onscreens-client publishes timewarp.show
3. timewarp.show graduates from the empty Command envelope to its own TimewarpShow{Username} payload — exactly what the Command doc comment always anticipated for "any event that grows a field"
4. onscreens-server's handleTimewarpShow decodes it and stores the username on the overlay's Content
5. the browser source reads Content on the showing rising-edge and renders @username, animating in with the wordmark

The credit line is hidden (via :empty CSS) when no username is attributable, so the warp still plays cleanly for any caller without a user — and the decode is lenient, so the old empty-envelope wire shape still triggers the warp (just with no credit).

Platform note

This is platform-agnostic plumbing — whatever username the chat command carries gets credited. In practice that's Twitch for now; if/when the YouTube bot runs these interactive commands, it'll credit YouTube chatters too with no further changes.

Tests

• onscreens-server: TestHandleTimewarpShow asserts the username lands on Content; new TestHandleTimewarpShow_NoUsername covers the empty-envelope back-compat path
• chatbot: the recordingOnscreens fake now captures the username; TestTimewarpCmd_* and TestGuessCmd_CorrectGuess_* assert the caller is credited
• onscreens-client: publish-subject test updated for the new signature

All four touched packages pass under task test:macos. (pkg/server has a pre-existing unrelated build failure — renderProfile undefined, left over from the panel removal in #886 — untouched here.)

Feature flag

The credit is gated behind the chatbot.timewarp_credit feature flag (evaluated in App.timewarp() with the standard EvalContext). The warp itself always runs — only the credit line is gated, so the flag can never affect playback.

• Seeded FALSE per platform in migration 021 (one row per platform, per the 019 convention), so the credit ships off and stays off until verified on stream.
• Because the flag client is platform-scoped (each bot loads only its STREAM_PLATFORM rows), this is exactly the lever for the "Twitch for now" note: enable chatbot.timewarp_credit on twitch, leave youtube off.
• New tests: TestTimewarpCmd_CreditFlagOff_NoUsername proves the warp still fires with no credit when the flag is off; the existing timewarp/guess tests enable the flag and assert the caller is credited.