Task: Coalesce a burst into one frame

A burst of changes reaches a client as one frame per entity, not one per row.

• [X] A burst of changes in one window becomes one frame per entity.
• [X] Two entities in one window become two frames.
• [X] A change is never attributed to another tenant or party.
• [X] A second window emits only what changed in it.

A dirty set keyed by tenant, party and entity, and a drain that swaps the map and turns each entry into a frame. The key carries the tenant and party because a frame is addressed at one party's data: folding two parties into one frame would tell a client to redraw rows it may not read.

The identifiers are a set, so a row written twice in one window is one dirty identifier, and a drained window is empty until something changes again.

• The burst case records 1000 rows and asserts one frame with 1000 ordered identifiers, which is the exit criterion of increment B3 stated as a case.
• A change with no entity or no identifier is dropped: it names nothing a client could redraw, and queueing it would make the frame lie about what changed.
• Nothing calls record yet. The feed that subscribes to the change events is the next task, and the tick that drains this belongs beside it.

The coalescer turns a burst into frames a screen can use: one per entity per window, addressed at one party, ordered, deduplicated, and empty once drained.