Lazy-load older PTY replay in web UI

[edwin-zvs]

Summary

• add offset metadata and optional max_bytes/before_offset params to PTY replay
• load only the newest 1 MiB of PTY history on first Web UI terminal attach
• when the user scrolls near the top, fetch only the next older PTY log chunk, keep chunks locally, and replay them in order
• show a terminal-history loading overlay while older PTY chunks are fetched/replayed

Tests

• cargo test -p agentd pty_range_before_returns_offsets_and_requested_slice
• cargo test -p agentd pty_range_before_clamps_to_file_start_and_end
• cargo test -p agentd pty_replay_returns_full_disk_tail_not_just_old_ring
• cargo test -p agentd pty_replay_caps_at_replay_max_for_huge_logs
• cargo check -p agentd
• cargo build
• git diff --check

Binaries

Built in this worktree under /Users/moon/agentd/.claude/worktrees/webui-lazy-pty-replay/target/debug/.
Relevant binary for this PR: agentd (crates/daemon, protocol + embedded Web UI assets).