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/package.json

@@ -50,6 +50,7 @@
     "directory": "packages/xterm-zerolag-input"
   },
   "devDependencies": {
+    "@xterm/headless": "^6.0.0",
     "jsdom": "^24.1.3",
     "tsup": "^8.5.1",
     "typescript": "^5.5.0",