Implements the stdio transport for the MCP server, enabling communication
with local agents (Claude Desktop, Claude Code, Continue, Cursor) over
standard input/output with Content-Length framing.
Core features:
- LSP-style Content-Length framing with \r\n terminators
- JSON-RPC 2.0 message parsing and serialization
- INV-9 compliance: stdout contains only JSON-RPC frames
- Panic hook redirects panics to stderr
- SIGTERM handler for graceful shutdown
- Parse errors return -32700 with id: null, then continue
Acceptance criteria:
- ✅ Piping tools/list with framing produces expected response < 50ms
- ✅ EOF on stdin → clean exit within 100ms
- ✅ Malformed JSON → -32700 error, subsequent requests work
- ✅ No println!/log output to stdout (INV-9 enforced)
- ✅ Panics go to stderr, no partial JSON on stdout
- ✅ SIGTERM → exit 0, SIGINT → immediate non-zero exit
Tests added:
- crates/pdftract-cli/tests/mcp-stdio.rs (8 integration tests, all pass)
- All 49 existing unit tests continue to pass
Refs: pdftract-67tm8, plan Phase 6.7.2
