fix(broker): isolate alternate-screen snapshots

[almogdepaz]

summary

• suppress primary-screen scrollback when broker snapshots serialize the active alternate screen
• strengthen the active-buffer regression so primary history exists before entering alt-screen
• keep primary-screen snapshot and restoration behavior unchanged

root cause

snapshot_with_reflow selected the alternate visible grid but unconditionally serialized inner.scrollback, which belongs to the primary screen. narrow viewports made the leak visible after reconnect because wrapped shell output had already entered primary scrollback.

verification

• regression observed red before the fix and green after
• Rust: 174 passed
• Bun: 1,695 passed with local Git signing disabled to match CI
• broker release build passed
• iphone-se broker TUI reconnect: 5/5 passed
• rustfmt and git diff --check passed
• standalone security review: approve, no findings

scope

independent of PR #180 and PR #183. no auth, protocol, PTY ownership, replay sequencing, or browser behavior changes.