Server components: a markup hole that re-emits re-renders every row of its list on every sweep #4056

A markup hole that emits more than once re-renders its whole value on every sweep. This applies to live addresses, plain responses reading a standing source, and the document face's sc:live ops. Server <For> / mapArray re-runs mapFn for every row on every pass, which is deliberate today (packages/solid/src/server/signals.ts, "never re-runs in isolation"). The sweep then ships the hole's full HTML unless it is byte-identical (packages/web/src/server.ts, sink.hole).

The result is that a list inside a streamed hole costs O(list) server render per write. Over a stream, that is O(list × writes).

Measured on examples/chat

Setup: simulated model, 12 KB replies, about 258 writes per reply, one tab, client CPU at 4x. The local instrumentation is a small patch, not committed.

• Each finished reply adds about 1.2 s of server CPU to every later reply. The finished-messages hole re-renders all of the history on every write, including markdown and highlighting, and then discards the result as identical. Each open tab pays this separately.
• The wire stays flat because the streaming brotli window absorbs re-sent prefixes. That only holds within one response and with a large window. With gzip's 32 KB window, a 60 KB hole re-sent per write measured 11.3 MB, versus 49 KB for an append-only stream.

Direction

Make server For behave like the client's in renders that can re-emit a hole:

• Keep rows by key across passes and reuse the last HTML of unchanged rows. Only new and changed rows render.
• Data needs stable identity across snapshots. For example, reconcile each snapshot into a store or projection, as the client does. Today watchThread clones every message on each write.
• The wire can then carry per-row ops (insert, replace, remove, move) instead of the whole hole. That is the "diff payload", with no string diffing needed.
• One-shot document SSR keeps today's path.

A growing single row (the message being written) still re-renders in full. Splitting it into keyed blocks would reuse the same mechanism.