Repaint sidebar after child scrolls the host scroll region

Codex (Ratatui) emits an 8x RI burst on startup right after setting
DECSTBM. RI at the top of the scroll region scrolls the region down,
and DECSTBM only constrains rows -- so the scroll spans every column
and drags the right-rail session-tree entries down with the main pane.
The chrome cache then hid the clobber because the computed sidebar
frame was unchanged.

The viewport renderer now flags any chunk containing RI / IND / NEL /
SU / SD / IL / DL and OnPTYOut drops the sidebar cache when the flag
is set, so the next drawSidebar repaints over the drift.

Adds unit tests for the new flag and a harness regression scenario
(sidebar_survives_ri_scroll) that fails without the fix.

internal/harness/scenarios/sidebar_survives_ri_scroll.json

@@ -0,0 +1,28 @@
+{
+  "name": "sidebar_survives_ri_scroll",
+  "cols": 80,
+  "rows": 24,
+  "scripts": [
+    {
+      "name": "riburst",
+      "body": "#!/bin/sh\n# Emulates codex's startup sequence: DECSTBM the full child\n# viewport, CUP to the top of the scroll region, and burst 8 RIs\n# (ESC M). RI at the top of the scroll region scrolls the region\n# down. The host's scroll region spans every column, so without\n# the sidebar cache-invalidation fix the right-hand session tree\n# gets dragged downward and the cache hides the broken state.\nprintf '\\033[1;21r'\nprintf '\\033[1;1H'\nprintf '\\033M\\033M\\033M\\033M\\033M\\033M\\033M\\033M'\nprintf '\\033[1;1HRIBURST READY\\n'\nsleep 5\n"
+    }
+  ],
+  "steps": [
+    {
+      "type": "mcp_call",
+      "method": "spawn_process",
+      "params": { "kind": "command", "argv": ["riburst"], "name": "riburst" }
+    },
+    { "type": "wait_text", "contains": "RIBURST READY", "timeout_ms": 5000 },
+    { "type": "wait_stable", "timeout_ms": 2000 },
+    { "type": "assert_contains", "contains": "Session tree" },
+    { "type": "assert_contains", "contains": "● riburst" },
+    { "type": "assert_contains", "contains": "Scratchpads" },
+    {
+      "type": "assert_regex",
+      "regex": "(?s)Session tree[^\\n]*\\n[^─\\n]*─[─]+[^\\n]*\\n[^●\\n]*● riburst",
+      "timeout_ms": 2000
+    }
+  ]
+}