Add regex support and negative assertions

[gnodet]

Summary

The current assertion/wait toolset has gaps that make it hard to test dynamic TUI content and verify that transient states (like loading spinners) have cleared.

Scope

1. Regex support in wait_for_text

Currently wait_for_text only does plain string matching (text in display). expect_text already supports regex patterns. For TUIs with dynamic content (timestamps, counters, IDs), regex matching on the live screen buffer would be useful.

Proposed API:

wait_for_text(text="Status: (OK|READY)", session_id="default", regex=True)

2. assert_not_contains tool

Negative assertion — verify that specific text is not present on screen. Useful for confirming error messages were cleared, dialogs were dismissed, etc.

Proposed API:

assert_not_contains(text="Error", session_id="default", use_buffer=None)

3. wait_for_text_gone tool

Wait for text to disappear from the screen buffer. Useful for testing loading states, progress indicators, and transient notifications.

Proposed API:

wait_for_text_gone(text="Loading...", session_id="default", timeout=10.0, poll_interval=0.2)

Acceptance criteria

• All three features implemented with proper docstrings
• Each feature has unit tests
• README and docs updated with examples