Control: replies are buffered, so a 17 MB reply is sent in 0.09 s instead of 34 s on Windows

The control server wrote each reply with writeln!(out, "{r}") on the
bare TcpStream. serde_json formats a Value one piece at a time, and
each piece (a brace, a quotation mark, a key, a colon, a number) was
sent as a write of its own: the 17 MB document.inspect reply of a
document with 50,000 objects on one layer was sent in 7,857,200 writes.
On Windows 11, sending that reply to a client over loopback took 34 s,
and sending the 28 MB document.json reply took 57 s.
vectorcraft-cli mcp reads every reply from a running app over this
connection.

write_line now writes each reply, each refusal and the "too many
control connections" line through a 64 KiB BufWriter, then flushes
it. The 17 MB reply takes 262 writes. Sending it now takes 0.09 s,
and sending the 28 MB reply takes 0.2 s. The bytes are the same.

Docs: docs/target-app-parity.md.

docs/target-app-parity.md

@@ -382,6 +382,7 @@ Moved from the ROADMAP's "Shipped so far" on 2026-10-10. Update it when a featur
   - Actions panel (record/playback, persisted), generic parameter dialogs for every "…" command.
   - Variables (data merge): text and visibility variables bound to objects (an object may carry several), dataset rows applied in one undo step (`variable.*`, `dataset.*`) from a Variables panel and Window › Variables, with dialogs to define, bind and edit, Make Text/Visibility Dynamic, Capture/Update Data Set, and a row in italics when an edit leaves the art not matching it; bindings let go of deleted art and locked art is left alone; every command, like the rest, is drivable over MCP and the CLI.
   - JSON-lines control channel with real egui pointer and keyboard injection.
+  - The control server writes each reply through a 64 KiB buffer. On Windows 11 it sends the 17 MB `document.inspect` reply of a document with 50,000 objects on one layer to a client over loopback in 0.09 s instead of 34 s.
   - MCP server with 25 tools (drawing, text, effects, Pathfinder, transforms, graphs, text wrap, export, screenshots, any command), attached to the running app or headless. Tool descriptions and replies agree with the engine: format lists come from its format table, `save_file` says it keeps the document's own format, `inspect_document` reports the paint type's characters show, and selection exports report the encoder's warnings. `--automation-read-root` / `--automation-write-root` (on `vectorcraft-cli mcp` and on `vectorcraft --control`) confine the files commands read and write to chosen folders, links followed, with the flags PhotoCraft takes (#832).
   - MCP protocol: five prompts (`prompts/list` / `prompts/get`: poster, icon set, recolour, trace-and-style, export set), `completion/complete` for prompt arguments and template variables read from the live catalogues (formats, effect ids, command ids, trace presets, swatch and object ids), four resource templates (`vectorcraft://object/{id}`, `//command/{id}`, `//effect/{id}`, `//swatch/{name}`) so an agent reads one thing instead of the whole document, and `logging/setLevel` with `notifications/message`. `window.panel` ids are case-insensitive (display labels work too), `vectorcraft://object/{id}` reads through `document.node {summary: true}` (the same reply bytes from one lookup instead of summarizing the whole document: a leaf read on a 2,000-object document drops from ~20 ms to ~0.06 ms), large containers slice with `document.node {depth?, childLimit?}` (truncated levels report `childCount`), `document.inspect` and the `inspect_document` tool slice the same way so a depth-0 read is the layer skeleton (~9.7 KB instead of ~2 MB on a 2,000-object document), `document.find` locates nodes by name/kind/text with ancestor paths, and the CLI sweeps the protocol surface over stdio against the compiled binary. `docs/mcp.md` has a Protocol section covering the capability table and what is deliberately absent (subscriptions, progress, cancellation, sampling, elicitation, Streamable HTTP — all of which need the transport to interleave reads).
   - Headless CLI (`vectorcraft-cli run`, `convert`, `info`, `bench`, `perf`, `mcp`). A `run` step, like a `command_batch` step over MCP, can use an earlier step's result (`"$1.id"`, #785).