feat: add xterm-zerolag-input standalone library

Extract Claudeman's local echo overlay into a reusable xterm.js addon
at packages/xterm-zerolag-input/. Provides instant keystroke feedback
via a DOM overlay, eliminating perceived input latency over high-RTT
connections (SSH, mobile, cloud IDEs).

- Zero dependencies, compatible with xterm v5.x and @xterm/xterm v5.4+
- Configurable prompt detection (character, regex, or custom function)
- Flushed text tracking for tab-switch / deferred echo scenarios
- Per-character grid-aligned rendering matching xterm's canvas output
- 56 tests passing (prompt finder, overlay renderer, full addon lifecycle)
- Dual CJS/ESM build with full TypeScript declarations

Claudeman source is unchanged — migration to consume this lib is a
separate follow-up.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

@@ -0,0 +1,11 @@
+export { ZerolagInputAddon } from './zerolag-input-addon.js';
+export type {
+    XtermTerminal,
+    XtermAddon,
+    ZerolagInputOptions,
+    ZerolagInputState,
+    PromptFinder,
+    PromptPosition,
+    CellDimensions,
+    FontStyle,
+} from './types.js';