Keep TUI zero state stable during shell startup

[warp-agent-staging]

Description

• Keep the empty TUI zero state vertically stable while the temporary Starting shell... hint is visible.
• Account for the hint row and its top padding only in the bootstrap zero-state slot; transcript, long-running-command, and GUI paths remain unchanged.
• Add a presenter-based 80×40 regression covering bootstrap and ready states.

Originating thread: https://warpdotdev.slack.com/archives/C0BDQDW8V5E/p1785652323329829

Linked Issue

• The request is tracked in APP-5098.
• Real live-TUI tmux capture-pane proof is included below.

Testing

• zero_state_position_stays_stable_across_shell_bootstrap failed before the fix and passes after it.
• bootstrap_renders_starting_shell_above_input and long_running_command_keeps_input_hidden pass.
• ./script/format --check
• cargo clippy -p warp_tui --all-targets --tests -- -D warnings
• cargo nextest run -p warp_tui --lib — 949/949 passed.
• cargo build -p warp_tui --bin warp-tui-oss
• cargo build -p warp_tui --bin warp-tui-dev

Full workspace presubmit completed format, inline-test, clippy, clang-format, and WGSL checks, but its workspace-test phase exceeded the sandbox target filesystem twice (No space left on device); the scoped TUI suite and PR CI are fully green.

Real live TUI proof

These frames came directly from tmux capture-pane while one authenticated warp-tui-dev process ran in an 80×40 PTY. They are terminal output from the real TUI, not replayed presenter buffers.

• Bootstrap frame — full terminal capture
• Ready frame from the same process — full terminal capture

Relevant lines (one-based capture-pane output; equivalent zero-based rows are 13/32/33):

bootstrap  14: Warp Agent CLI
bootstrap  33: Starting shell...
bootstrap  34: ▁▁▁...  (input top border)
ready      14: Warp Agent CLI
ready      34: ▁▁▁...  (input top border)

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-TUI: Fixed the zero state shifting when shell startup completes.

Rework changes

• ⚠️ Real TUI proof: installed the pinned internal channel-config generator, built the dev/dogfood TUI, authenticated with the inherited cloud-runner API key, and captured bootstrap plus ready frames from the same live tmux session. The title and input border remain on identical rows while Starting shell... disappears.
• No code changes were needed; the review finding concerned verification evidence only.

Conversation: https://staging.warp.dev/conversation/5020fa91-cade-4b17-8d57-af8c5de70f0c
Run: https://oz.staging.warp.dev/runs/019fc139-4de5-70c1-a4ef-4a772853163a

Co-Authored-By: Oz oz-agent@warp.dev
This PR was generated with Oz.