fix(tui): retire finished rows before viewport drop under pinned frontier

[F0Rextasy]

What

I reviewed the full diff; overflow rows clipped off the live viewport vanished without ever reaching native scrollback while a Task block pinned retirement, so the viewport now synchronously retires finished head rows before applying the drop slice.

Why

Fixes #12584

Testing

• bun test packages/tui/test/transcript-container.test.ts (32 pass, 1 new: finished head retires before pinned-frontier clip)
• bun --cwd=packages/tui run check:types (pass)
• oxlint + oxfmt on both changed files (pass)

---

• bun check passes
• Tested locally
• CHANGELOG updated with the required attribution (if user-facing; internal issue fixes use issue links, external contributions add the PR link and contributor credit after creation)

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