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/cell-dimensions.ts

@@ -0,0 +1,36 @@
+import type { XtermTerminal, CellDimensions } from './types.js';
+
+/**
+ * Get cell dimensions from the terminal, handling xterm.js v5 (private API)
+ * and v7+ (public API).
+ *
+ * Returns `null` if the terminal is not yet rendered or dimensions are
+ * unavailable.
+ */
+export function getCellDimensions(terminal: XtermTerminal): CellDimensions | null {
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    const t = terminal as any;
+
+    // Try v7+ public API first
+    if (t.dimensions?.css?.cell) {
+        return {
+            width: t.dimensions.css.cell.width,
+            height: t.dimensions.css.cell.height,
+        };
+    }
+
+    // Fall back to v5 private API
+    try {
+        const dims = t._core?._renderService?.dimensions;
+        if (dims?.css?.cell) {
+            return {
+                width: dims.css.cell.width,
+                height: dims.css.cell.height,
+            };
+        }
+    } catch {
+        // Private API may throw in some environments
+    }
+
+    return null;
+}