[dev-rehaann]

Introduces forensic chain format version 2 with an explicit event_type discriminator.

• ingestion_accepted records contain collector-specific fields and omit all ML-only fields
• classification_decision records retain the ML schema
• Python and Rust verifiers accept unmixed legacy v1 chains and typed v2 chains
• mixed-version append and verification are rejected
• the collector adapter no longer fabricates ML placeholder values
• the 25 float-canonicalization vectors and canonical.rs are unchanged

This branch includes the collector-forensics adapter work from PR #3 and is intended to supersede PR #3 after review. It does not include PR #2.