feat(predictive-echo): Phase 0 codex fixtures, measurements, package scaffolding

Recorder (scripts/dev/record-codex-frames.mjs) captures real codex 0.147
TUI output through the production pipeline (tmux status-off + the codex-mode
full strip from session.ts) into JSONL fixtures with keystroke injection
points; analyzer replays them through @xterm/headless for the measurements
in docs/predictive-echo-plan.md. Composer signature /^> /-style (U+203A),
modal and wrapped rows correctly rejected, echo is unstyled default-fg,
tmux delivers echo as minimal in-place deltas.

Package: types.ts gains optional cursorX/cursorY, getCell, onWriteParsed,
onResize (all additive); prediction-renderer.ts renders per-glyph spans
keyed by prediction seq; @xterm/headless@^6.0.0 devDep for replay tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packages/xterm-zerolag-input/src/prediction-renderer.ts

@@ -0,0 +1,59 @@
+/**
+ * Incremental DOM renderer for PredictiveEchoAddon.
+ *
+ * Unlike overlay-renderer.ts (which paints whole lines with an opaque
+ * background out to totalCols), prediction spans cover ONLY the predicted
+ * glyph's own cells: anything wider would blank real echo arriving around
+ * a prediction. Spans are keyed by prediction seq for O(1) removal.
+ */
+import type { CellDimensions, FontStyle } from './types.js';
+
+export interface PredictionSpanParams {
+  seq: number;
+  /** Viewport-relative row (0-based). */
+  row: number;
+  /** Column (0-based). */
+  col: number;
+  char: string;
+  /** Cell width of the glyph (1 or 2). */
+  width: 1 | 2;
+  dims: CellDimensions;
+  font: FontStyle;
+  underline: boolean;
+}
+
+export function addPredictionSpan(
+  container: HTMLElement,
+  map: Map<number, HTMLSpanElement>,
+  p: PredictionSpanParams
+): void {
+  const span = document.createElement('span');
+  // cellH+1 height: covers the sub-pixel seam between rows (same trick the
+  // buffer overlay renderer ships with). Background covers only this glyph's
+  // cells, never a full row.
+  span.style.cssText =
+    `position:absolute;left:${p.col * p.dims.width}px;top:${p.row * p.dims.height}px;` +
+    `width:${p.width * p.dims.width}px;height:${p.dims.height + 1}px;line-height:${p.dims.height}px;` +
+    `text-align:center;pointer-events:none;` +
+    `font-family:${p.font.fontFamily};font-size:${p.font.fontSize};font-weight:${p.font.fontWeight};` +
+    (p.font.letterSpacing ? `letter-spacing:${p.font.letterSpacing};` : '') +
+    `color:${p.font.color};background-color:${p.font.backgroundColor};` +
+    `font-feature-settings:'liga' 0,'calt' 0;` +
+    (p.underline ? 'text-decoration:underline;' : '');
+  span.textContent = p.char;
+  map.set(p.seq, span);
+  container.appendChild(span);
+}
+
+export function removePredictionSpan(map: Map<number, HTMLSpanElement>, seq: number): void {
+  const span = map.get(seq);
+  if (span) {
+    span.remove();
+    map.delete(seq);
+  }
+}
+
+export function clearAllSpans(map: Map<number, HTMLSpanElement>): void {
+  for (const span of map.values()) span.remove();
+  map.clear();
+}

packages/xterm-zerolag-input/src/types.ts

@@ -22,9 +22,15 @@ export interface XtermTerminal {
     readonly active: {
       readonly viewportY: number;
       readonly baseY: number;
+      /** Cursor column (0-based). Used by PredictiveEchoAddon. */
+      readonly cursorX?: number;
+      /** Cursor row, relative to baseY (0-based). Used by PredictiveEchoAddon. */
+      readonly cursorY?: number;
       getLine(y: number):
         | {
             translateToString(trimRight?: boolean): string;
+            /** Cell access (xterm public API). Optional: mocks/exotic hosts may omit it. */
+            getCell?(x: number): { getChars(): string; getWidth(): number } | undefined;
           }
         | undefined;
     };
@@ -34,6 +40,10 @@ export interface XtermTerminal {
     getStringCellWidth(str: string): number;
     activeVersion?: string;
   };
+  /** Fires after the parser finishes a write chunk. Used by PredictiveEchoAddon. */
+  onWriteParsed?(cb: () => void): { dispose(): void };
+  /** Fires on terminal resize. Used by PredictiveEchoAddon. */
+  onResize?(cb: (size: { cols: number; rows: number }) => void): { dispose(): void };
 }
 
 /**