Crossterm queries cursor position during initialization via ESC[6n control
sequence, but the PTY emulator doesn't automatically respond. This caused
tests to timeout waiting for the response.

Added intercept_control_sequences() to detect cursor position queries in
the PTY output stream and write back ESC[1;1R responses, enabling the TUI
to initialize properly in the test environment.

Updated test expectations to match the initial welcome screen that now
appears, and added snapshot for regression detection.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

…imeout hangs

Previously, tests using wait_for_text() would hang indefinitely when the
expected text never appeared, because the PTY reader was in blocking mode.
This caused read() to block forever waiting for data, preventing the timeout
mechanism from executing.

Changes:
- Set PTY master to non-blocking mode using fcntl(O_NONBLOCK) on Unix
- Add helper function set_nonblocking() using libc for low-level fcntl ops
- read() now returns WouldBlock immediately when no data is available
- Timeout checks in wait_for() can now execute, failing tests cleanly after 5s
- Add test_poll_does_not_block_when_no_data to verify non-blocking behavior
- Convert debug logging to conditional (DEBUG_TUI_PTY env var)
- Uncomment and fix previously disabled tests in startup.rs
- Update documentation with PTY non-blocking details and debugging info

Dependencies added (Unix only):
- nix = "0.27" (features: fs)
- libc = "0.2"

The fix ensures test timeouts work correctly, with tests failing cleanly
rather than hanging indefinitely when conditions are not met.
🤖 Generated with [Nori](https://nori.ai)

Co-Authored-By: Nori <noreply@tilework.tech>

…ession testing

Adds snapshot assertions to 4 startup tests to capture final screen states:
- startup_shows_welcome: Welcome screen with logo and /tmp/ path
- startup_welcome_dimensions_40x120: Welcome screen at 40x120 dimensions
- runs_in_temp_directory: Verification of /tmp/ directory usage
- trust_screen_skipped: Main prompt without trust screen

Excludes test_poll_does_not_block_when_no_data as it tests timing behavior, not UI.
🤖 Generated with [Nori](https://nori.ai)

Co-Authored-By: Nori <noreply@tilework.tech>

…overage

Move normalize_for_snapshot() and TIMEOUT constant from test modules to library exports
for consistent reuse across all tests. Consolidate streaming/cancellation tests into a
unified streaming.rs module.

Add Drop trait implementation to TuiSession that prints screen state on panic, improving
debugging of PTY timing issues. Expand snapshot testing coverage to input handling
scenarios (ctrl-c, typing, arrow navigation).

Add 100ms sleep delays after PTY input operations in tests to prevent race conditions
between sending input and TUI processing. Update documentation to reflect new exported
utilities, debugging aids, and PTY timing patterns.