fix(tui): add MAX_WRAP_INPUT_LEN guard to prevent UI freeze on oversized lines (closes #26)

- compute_line_entry/compute_visual_height: double guard (raw + post-format)
- Skip detect_level on oversized raw input to avoid O(n) JSON parsing
- Add post-format guard for JSON lines that expand beyond 10MB
- progressive_reader: add post-format guard to compute_line_visual_height
- Add truncate_to_columns helper using existing wrap_line_chars
- Fix misleading docstring on MAX_WRAP_INPUT_LEN constant
- Add 6 regression tests covering all guard paths

crates/core/src/io/wrap.rs

@@ -1,5 +1,6 @@
 /// Maximum input length for wrap/format operations (10 MB).
-/// Lines exceeding this are returned as-is to avoid pathological cases.
+/// Callers should check against this constant before invoking `wrap_line_chars`
+/// to avoid pathological cases on oversized lines.
 pub const MAX_WRAP_INPUT_LEN: usize = 10 * 1024 * 1024;
 
 /// Split a line into chunks of exactly `width` display columns.