fix(io): cap line reads on the serve and MCP transports

Bug:
The --serve automation loop (stdin/socket) and the MCP stdio loop read
input with unbounded read_line/lines. A single line over 64 MiB - a
huge paste or a hostile client - grows the buffer until the allocator
aborts, the same class of failure as the HTTP path, on the non-HTTP
transports.

Reproduce:
1. Start --serve on a socket (or the MCP server on stdio).
2. Send one 65 MiB line with no terminating newline anywhere in it.
3. Before: RSS grows to the full line, then "memory allocation failed"
   (or a multi-second stall while the line is copied).
4. After: the read fails at the cap and the existing EOF arm closes the
   connection/stdio promptly.

Fix:
New shared src/io/line_read.rs module: lines_capped(reader, 64 MiB)
yields Err for an over-cap line, which the serve and MCP loops already
treat as EOF/close. rest.rs was refactored onto the same helpers so the
cap logic lives in one place. A regression test streams a 65 MiB line
at the serve socket and asserts the connection ends without a response.

src/io/mod.rs

@@ -20,6 +20,7 @@ pub mod stl;
 pub mod xref;
 pub mod xref_model;
 pub mod linetypes;
+pub mod line_read;
 pub mod patterns;
 pub mod update_check;
 pub mod paper_catalog;