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.
