perf(engine): cut live-stream burst volume — opt-in sync copies, patches on demand, throttled shell output #1778

Acceptance Criteria

• The global stream omits sync copies unless the subscriber requests them; the workspace control-plane subscription requests them and still replays correctly.
• Summary diffs in live events carry file, status, additions, and deletions but no patch bodies; the stored record is unchanged.
• Expanding a file in the per-turn diff view fetches and renders its patch; diff counts and file lists render as before without any fetch.
• A shell command printing continuously produces at most about five output updates per second on the stream, and its final output is always delivered on completion.
• A fixture turn (multi-file edit plus a chatty shell command) keeps its largest live frame under 256 KB, and the slice reports total stream bytes before and after.

Testing Decisions

• Extend the engine server suite (opt-in filter, wire projection), the shell tool tests (throttle and final flush), and the session-ui tests for the per-turn diff view (on-demand fetch).

Key Decisions

• Sync copies are opt-in per subscription, off by default.
• The projection is wire-only; storage is untouched.
• Throttle: the first update goes out immediately, then at most one per 200 ms with a trailing flush.

Constraints & Invariants

• Sync copies keep their source event's id, so filtering them leaves no sequence holes.
• Nothing about what is persisted changes.

Prior Art

• The event-v2 bridge's twin emission; the control-plane workspace's replay of sync events; the session summary diff shape; the shell tool's output preview; the per-turn diff accordion and the diff-change bars.

Source

• Part of Fleet event stream is lossy by construction — remote sessions lag and desync during live activity #1770 · Blocked by feat(engine): sequenced, replayable global event stream (epoch:seq ids, replay buffer, reset) #1776 (shares the global stream handler).