docs(pdftract-3b1x): finalize sdk-invocation.md with subprocess contract and TH-07 compliance
Add comprehensive "Subprocess Contract" section documenting:
- argv layout with canonical form
- stdin discipline (password ingress, PDF bytes from stdin)
- stdout/stderr discipline (what goes where, what never gets logged)
- Exit code taxonomy (0, 64-78) with TH-03 (exit 78) and TH-07 (exit 64) refs
- Environment variable pass-through (PDFTRACT_PASSWORD, PDFTRACT_MCP_TOKEN, etc.)
- --progress-json event schema (ndjson format, all event types)
- --capture-diagnostics archive layout (zip/tar, contained files, scrubbing rules)
Update all language examples (Python, Node.js, Go, Ruby, Java, Rust) with
TH-07-compliant password handling:
- Pass password via PDFTRACT_PASSWORD env var (subprocess)
- Pass password via multipart form field (HTTP)
- Never use --password VALUE flag (rejected unless opt-in)
Add progress JSON parsing examples for Python, Node.js, and Rust showing
real-world event-driven progress tracking.
File grows from 1100 to 1837 lines (+737 lines, ~67%).
Closes: pdftract-3b1x
57df42f478