Update TUI visibility pruning incrementally

[jmqd]

Each approve/reject locally prunes visibility by rebuilding visible ancestors from remaining visible blocks. Audit refs: trueflow/src/commands/tui.rs:3908, trueflow/src/commands/tui.rs:3930, trueflow/src/review_navigator.rs:95, trueflow/src/review_navigator.rs:157. Trigger: large review with one-by-one approvals. Current shape is O(remaining * depth) per action. Direction: maintain visible descendant or ancestor reference counts and update incrementally.

[jmqd]

Patch: 27fb763 replaces per-action full visible-ancestor rebuilds with incremental visible block descendant counts in ReviewNavigator. Approve/reject removes blocks in O(depth); child expansion replaces the parent block contribution with child block contributions. Verification: cargo test review_navigator::tests --lib; cargo test review_session::tests --lib; cargo test approve_action_prunes_visible_ancestors_after_last_block --lib; cargo test handle_child_lazily_expands_small_markdown_section_into_inclusive_sections --lib; just check passed with 994 tests, clippy, and fmt.

[jmqd]

Follow-up patch: cb3c16a preserves visible descendant traversal order after the incremental pruning change. The regression test locks sibling order for visible block descendants so TUI counts/navigation do not flip file-order siblings while pruning remains incremental. Verification: cargo test --manifest-path trueflow/Cargo.toml review_navigator::tests --lib; just check passed with 994 tests, clippy, and fmt.