Add user and session investigations and verified installation

[kwisatzh]

Summary

• Add user/session contribution questions with bounded shares and deltas, extraction-contract compatibility checks and conservative session review flags.
• Treat absent optional measurements as unavailable, not zero; suppress flags when relevant observations are incomplete.
• Use exact decimal threshold comparison, including large-counter boundary cases.
• Add a verified installer that checks attestation and checksum before extraction.
• Improve quickstarts and installation guidance. Replace historical benchmark numbers with a reproducible public v0.2.0 rerun and checked-in output.

Validation

• Go race tests and vet pass, including installer and contributor tests.
• Single-app, missing-usage and two-stack demos passed.
• Published-tag resource harness passed valid and malformed-input checks.
• Public-file privacy scan and whitespace checks pass.

New attribution features require the matching collector configuration and are not claimed for existing release binaries. No release or tag is included.