## Why
The TUI app module had grown past the 512K source-file cap enforced by
CI/CD. This keeps the app entry point below that limit while preserving
the existing runtime behavior and test surface.
## What changed
- Kept the top-level `App` state and run-loop wiring in
`tui/src/app.rs`.
- Split app responsibilities into focused private submodules under
`tui/src/app/`, covering event dispatch, thread routing, session
lifecycle, config persistence, background requests, startup prompts,
input, history UI, platform actions, and thread event buffering.
- Moved the existing app-level tests into `tui/src/app/tests.rs` and
reused the existing snapshot location rather than adding new tests or
snapshots.
- Added module header comments for `app.rs` and the new submodules.
## Follow-up
A future cleanup can move narrow unit tests from `tui/src/app/tests.rs`
into the specific app submodules they exercise. This PR keeps the
existing app-level tests together so the refactor stays focused on the
source-file split.
## Verification
- `cargo test -p codex-tui --lib
app::tests::agent_picker_item_name_snapshot`
- `cargo test -p codex-tui --lib app::tests::clear_ui`
- `cargo test -p codex-tui --lib
app::tests::ctrl_l_clear_ui_after_long_transcript_reuses_clear_header_snapshot`
- `just fix -p codex-tui`
Full `cargo test -p codex-tui` still fails on model-catalog drift
unrelated to this refactor, including stale
`gpt-5.3-codex`/`gpt-5.1-codex` snapshot and migration expectations now
resolving to `gpt-5.4`.
Eric Traut