feat: add @wterm/serialize for session persistence

Closes #35.

Adds a new package @wterm/serialize that snapshots wterm state (grid cells, scrollback, cursor, modes, palette) into a portable string and restores it into a fresh terminal. Matches the capability xterm.js users get from @xterm/addon-serialize.

Usage

import { WTerm } from "@wterm/dom";
import { serialize, restore } from "@wterm/serialize";

const term = new WTerm();
term.open(document.getElementById("terminal")!);
term.write("some output\r\n");

const snapshot = serialize(term);
localStorage.setItem("wterm-session", snapshot);

const next = new WTerm();
next.open(document.getElementById("terminal")!);
restore(next, localStorage.getItem("wterm-session")!);

Approach

• Snapshot is an ANSI stream: replaying it via term.write() reproduces the exact cell state, which reuses the renderer you already have rather than introducing a parallel restore path
• Encodes visible grid + scrollback + cursor position + SGR attributes + DECSET modes + 256/truecolor palette
• Zero runtime deps
• New package only; no edits to @wterm/core or @wterm/dom

Testing

17 new tests in packages/@wterm/serialize/src/__tests__/:

• encode: SGR attrs, 256-color, truecolor, scrollback, DECSET modes, cursor position
• roundtrip: serialize then restore produces an identical grid snapshot

Full repo CI is green: pnpm test 151 passing, pnpm format:check, pnpm typecheck, pnpm build all clean.

Demo

Left terminal holds a live session. serialize() returns a snapshot string. A fresh WTerm mounts after reload and restore() rebuilds the exact state into the right terminal.

Notes

• Docs page and API reference entry added per AGENTS.md
• No new runtime dependencies
• Ready for review

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