fix: cap control requests and refuse file reads that never end

The control channel read each request with no length cap, so a client sending
gigabytes without a newline made the server thread allocate until the process
was killed. Read lines against the 16 MiB cap the MCP server already uses, now
one shared constant, answer a longer one with an error and close; close a
connection after eight lines that are not JSON, serve at most sixteen
connections at once, and run at most sixty-four queued requests per frame so a
flood cannot starve drawing.

Every file command checked the size with the metadata and then read to the end.
A character device such as /dev/zero reports a length of 0, so the cap passed
and the read was unbounded; a FIFO blocked the calling thread, which over the
control channel is the thread drawing the window. vfs::read now takes the
caller's cap, refuses anything that is not a regular file before opening it,
re-checks on the handle to close the window between the two, and reads no more
than the cap plus the byte that proves it was exceeded. Callers pass the limits
they already had, and the recovery read and the CLI's own reads go through it
too. `solvecraft-cli eval /dev/zero` and a FIFO now fail at once instead of
never returning.

Fixes #147

Co-Authored-By: Claude Opus 5 <[email protected]>
(cherry picked from commit 4156e8acd11c90c435a54a3b0ccc59991db71b12)

docs/control-protocol.md

@@ -6,6 +6,14 @@ line `{"id": …, "ok": true, "result": …}` or `{"id": …, "ok": false, "erro
 handled on the UI thread between frames; pointer and keyboard input is injected as real egui
 events, one pointer event per frame.
 
+The channel is untrusted input. A request line may be at most 16 MiB; a longer one is answered
+with `{"ok": false, "error": "request too large"}` and the connection is closed, as are
+connections that send eight lines that are not JSON. At most 16 connections are served at once
+(further ones get `{"ok": false, "error": "too many connections"}`), at most 64 queued requests
+run per frame so a flood cannot starve drawing, and a request that the UI thread has not
+answered within 60 seconds is replied to with `{"ok": false, "error": "timeout"}`. The same
+16 MiB line limit applies to the MCP server.
+
 ## Engine
 
 | Method | Params | Result |