Polish chrome and rework tab-switch repaint

Module renamed github.com/harrybrwn/patterm → github.com/hjbdev/patterm
across imports.

Chrome:
- Palette redrawn with rounded box-drawing borders, accent left-bar
  for the selected item, dim hints, and a separator-aware footer.
- Tab bar grew from 1 row to 3: labels with breathing room, a dim
  argv subtitle truncated to each tab's width, and an accent thick
  underline for the focused tab with a faint divider extending across
  the rest of the host width. Layout, viewport-renderer, and screen-
  renderer tests updated for the new mainTop.
- Sidebar reuses the same palette: accent section headers, `▎`
  selection marker, `●`/`○` status glyphs, dim previews.
- Shared SGR constants moved into internal/app/style.go.

Palette input:
- Adjacent duplicate arrow events (legacy `\x1b[B` + kitty
  `\x1b[57353u` for one keypress, or two of the same form) are now
  collapsed via peekArrowEvent + chunk-level dedupe in processStdin.
- On open, push `\x1b[>0u` onto the host's kitty keyboard stack so
  palette input is in plain legacy mode regardless of what the child
  pushed (codex/ratatui pushes its own flags which had been leaking
  to the host). Popped on close.

Tab-switch repaint (repaintFocused):
- Use the emulator's SerializeVT bytes (with SGR / cursor / DECSTBM
  / tabstops) instead of plain text, fed through the per-focused
  viewport renderer so the shifter translates row positions.
- Prelude resets host SGR / DECOM / DECSTBM (pinned to viewport) /
  cursor visibility before the replay, so leftover modes from the
  previously-focused child don't distort the new snapshot.
- Re-emit the saved cursor as a child-space CUP after the
  serialized bytes so the host cursor lands at the emulator's
  actual position (overriding DECSTBM's home side-effect and the
  tabstop-setup CHA sequences) AND the renderer's vr.row/vr.col
  get re-synced via trackCSI.
- cursorShifter now carries childRows and rewrites empty
  `\x1b[r` to `\x1b[<mainTop>;<mainBottom>r` (host coords) — the
  default (1,1) shifted to (4,4) was producing a one-row scrolling
  region that scroll-exploded the replay.
- After the snapshot lands, nudge the focused child with a one-row
  PTY winsize toggle so the kernel emits SIGWINCH and ratatui-style
  TUIs throw away their diff state and emit a fresh frame.

Codex still renders incorrectly after a focus switch; see TODO.md
"Switch-back render divergence" for the deep investigation handoff.

internal/app/cursorshift_test.go

@@ -6,7 +6,7 @@ import (
 )
 
 func TestCursorShifterCUP(t *testing.T) {
-	cs := newCursorShifter(1)
+	cs := newCursorShifter(1, 36)
 	got := cs.Shift([]byte("\x1b[H"))
 	want := []byte("\x1b[2;1H")
 	if !bytes.Equal(got, want) {
@@ -15,7 +15,7 @@ func TestCursorShifterCUP(t *testing.T) {
 }
 
 func TestCursorShifterCUPRowCol(t *testing.T) {
-	cs := newCursorShifter(1)
+	cs := newCursorShifter(1, 36)
 	got := cs.Shift([]byte("\x1b[10;5H"))
 	if string(got) != "\x1b[11;5H" {
 		t.Fatalf("CUP 10;5: got %q", got)
@@ -23,7 +23,7 @@ func TestCursorShifterCUPRowCol(t *testing.T) {
 }
 
 func TestCursorShifterVPA(t *testing.T) {
-	cs := newCursorShifter(1)
+	cs := newCursorShifter(1, 36)
 	got := cs.Shift([]byte("\x1b[7d"))
 	if string(got) != "\x1b[8d" {
 		t.Fatalf("VPA 7: got %q", got)
@@ -31,15 +31,27 @@ func TestCursorShifterVPA(t *testing.T) {
 }
 
 func TestCursorShifterDECSTBM(t *testing.T) {
-	cs := newCursorShifter(1)
+	cs := newCursorShifter(1, 36)
 	got := cs.Shift([]byte("\x1b[2;20r"))
 	if string(got) != "\x1b[3;21r" {
 		t.Fatalf("DECSTBM: got %q", got)
 	}
 }
 
+// Empty DECSTBM (\x1b[r) is a reset request; without a viewport-aware
+// fix it would default to (1,1) and shift to a one-row scrolling
+// region — that's what was scrolling claude's content up after a
+// focus switch from codex.
+func TestCursorShifterDECSTBMEmptyResetsToViewport(t *testing.T) {
+	cs := newCursorShifter(3, 36) // mainTop=4, childRows=36
+	got := cs.Shift([]byte("\x1b[r"))
+	if string(got) != "\x1b[4;39r" {
+		t.Fatalf("empty DECSTBM reset: got %q want \\x1b[4;39r", got)
+	}
+}
+
 func TestCursorShifterPrivateCSIPassthrough(t *testing.T) {
-	cs := newCursorShifter(1)
+	cs := newCursorShifter(1, 36)
 	// Alt-screen toggle — private CSI.
 	got := cs.Shift([]byte("\x1b[?1049h"))
 	if string(got) != "\x1b[?1049h" {
@@ -48,7 +60,7 @@ func TestCursorShifterPrivateCSIPassthrough(t *testing.T) {
 }
 
 func TestCursorShifterSGRPassthrough(t *testing.T) {
-	cs := newCursorShifter(1)
+	cs := newCursorShifter(1, 36)
 	got := cs.Shift([]byte("\x1b[1;31mhello\x1b[0m"))
 	if string(got) != "\x1b[1;31mhello\x1b[0m" {
 		t.Fatalf("SGR: got %q", got)
@@ -56,7 +68,7 @@ func TestCursorShifterSGRPassthrough(t *testing.T) {
 }
 
 func TestCursorShifterStraddleChunks(t *testing.T) {
-	cs := newCursorShifter(1)
+	cs := newCursorShifter(1, 36)
 	a := cs.Shift([]byte("\x1b["))
 	b := cs.Shift([]byte("5;3H"))
 	got := string(a) + string(b)
@@ -66,7 +78,7 @@ func TestCursorShifterStraddleChunks(t *testing.T) {
 }
 
 func TestCursorShifterOSCNotRewritten(t *testing.T) {
-	cs := newCursorShifter(1)
+	cs := newCursorShifter(1, 36)
 	// OSC body containing what looks like a CSI cursor move — should
 	// NOT be rewritten.
 	in := []byte("\x1b]0;\x1b[5;3Htitle\x07")