feat(tui): add session render state dump

[kitlangton]

Summary

• add a command-palette-only Dump session render state action for local, dev, and beta builds
• write session message/part text lengths, renderable bounds, and scroll position to a JSON snapshot in the working directory
• invoke the existing OpenTUI buffer dump at the same timestamp for comparison with terminal output

Purpose

This is diagnostic instrumentation for investigating intermittent blank session regions. It distinguishes missing application state from detached scrolling and from OpenTUI buffer/output failures. It is not enabled in the latest release channel.

Verification

• bun typecheck in packages/opencode
• bunx oxlint packages/opencode/src/cli/cmd/tui/routes/session/index.tsx (existing warnings, no errors)
• manually invoked through the existing TUI command event path in a local build; it emitted session-<id>-render-state-<timestamp>.json and matching buffer_dump/*_<timestamp>.txt files