feat: replay scrollback history on attach

[ttak0422]

Summary

Fixes the "cannot scroll" bug: attaching to a session only rebuilt the visible screen via STATE_SYNC, so after a Neovim restart the terminal buffer had no scrollback — normal-mode scrolling, search, and markers found nothing above the viewport.

Implements proposals 0001 and 0002 (included under docs/proposals/):

• Proto: PROTO_VERSION = 1, client HELLO (version + flags), server HELLO_ACK, server HISTORY, plus encode/parse helpers.
• Session: history_formatted() pages through the vt100 scrollback with set_scrollback and emits history lines oldest-first as a raw escape-sequence stream. Wrapped rows are emitted without \r\n so long lines re-join in the terminal buffer and search works across wrap points. Ends with one viewport of padding so the following STATE_SYNC redraw does not overwrite the history tail. Empty while the alternate screen is active.
• Server: records proto/wants_history from HELLO; queues HISTORY once, just before the initial STATE_SYNC. REDRAW and later RESIZE re-snapshots never resend history, so auto_redraw (which fires on every BufEnter) cannot duplicate it. HELLO_ACK is (re)queued ahead of the first STATE_SYNC even when replace_send_buf drops queued frames; duplicate ACKs are idempotent. Replay length capped by PTERM_HISTORY_REPLAY_LINES (default: unlimited).
• Bridge: sends HELLO + RESIZE in a single write, writes HISTORY payloads to stdout like OUTPUT, and prints a one-line notice on protocol mismatch (a daemon that never sends HELLO_ACK before the first STATE_SYNC is treated as v0).

Backward compatible in both directions: clients or daemons without HELLO keep the legacy behavior.

Proposals 0003 (daemon handoff) and 0004 (reboot persistence) are recorded as withdrawn.

Test plan

• cargo test: 37 + 9 proto tests pass, including new coverage for history content/no-duplication/max_lines/wrapped re-join/altscreen (session), frame ordering HELLO_ACK → HISTORY → STATE_SYNC, once-only history, and legacy-client behavior (server), and hello/hello_ack roundtrips (proto).
• E2E with the release binary: attached to a session holding 200 printed lines; observed history from line 1, viewport padding, then the STATE_SYNC redraw, with no protocol notice and clean detach.

🤖 Generated with Claude Code