ROOT CAUSE: When child windows used ImVec2(0, 0) for auto-fill, the
child's reported height was unstable inside tab items (especially when
the parent tab was inside a tab_bar inside a window). Result: the
scrollable child rendered with a fixed smaller height, showing only the
first half of the content, with empty space below.
FIX: Use imgui.get_content_region_avail() to compute explicit dimensions
and pass them to begin_child. Now the child fills the full available area
inside the tab content.
- render_comms_history_panel: avail.x, avail.y
- render_prior_session_view: same, plus added entry count indicator next
to the Exit Prior Session button ({N} entries) for at-a-glance info
Tests:
- test_comms_scroll_no_clipping.py: verifies comms_scroll child uses
explicit (non-zero) size
- test_prior_session_no_clipping.py: same for prior_scroll child
- test_log_management_first_open.py: minor cleanup
- 42/42 broad regression pass
