fix decompiler highlighting + cursor jank

Highlighting: Textual has NO C/C++ tree-sitter grammar (and the syntax extra
wasn't installed), so language='cpp' was a silent no-op. Replace TextArea with a
virtualized, Pygments-highlighted ScrollView:
- idatui/highlight.py: CLexer -> Rich styles, per-line Segment lists
- DecompView: lines highlighted ONCE at load, cached as Strips; O(1) scroll

Jank: profiling showed our code is ~3.6ms/move (render_line 0.018ms) -- the cost
was terminal repaint volume, since every cursor move refreshed the whole ~43-line
viewport. Now:
- cursor reactive repaint=False (no implicit full refresh)
- region-limited refresh: in-place moves repaint only the 2 changed rows
  (measured 43 -> 2 render_line calls/move), full refresh only on scroll
Applied to both DisasmView and DecompView.

pilot suite 15/15 (adds highlighting assertion).

1 files changed, 8 insertions, 2 deletions

diff --git a/tests/test_tui.py b/tests/test_tui.py
index 683f896..0054765 100644
--- a/tests/test_tui.py
+++ b/tests/test_tui.py
@@ -135,8 +135,14 @@ async def run(db):
         pc = await wait_until(pilot, lambda: dec.display and dec.loaded_ea is not None, 25)
         check("tab shows pseudocode", pc and app._active == "decomp",
               f"active={app._active} disp={dec.display}")
-        check("pseudocode has real body (not 1KB stub)", len(dec.text) > 1200,
-              f"len={len(dec.text)}")
+        check("pseudocode has many lines", dec.total > 20, f"lines={dec.total}")
+        # Highlighting: at least one styled (colored) segment across the body.
+        styled = any(
+            seg.style is not None and seg.style.color is not None
+            for strip in dec._strips[: min(dec.total, 200)]
+            for seg in strip
+        )
+        check("pseudocode is syntax-highlighted", styled)
         await pilot.press("shift+tab")
         await pilot.pause(0.2)
         check("shift+tab returns to disassembly",