V-L2-C3: positive tamper-detection tests for actor / before_snapshot / transformation

[hyperpolymath]

Context

Once V-L2-C1 lands, tampering with actor, before_snapshot, or
transformation must break verify(). Today only operation,
entity_id, timestamp, and previous_hash are protected.

What to do

In src/abi/mod.rs::tests add three parallel positive cases:

#[test]
fn test_provenance_tamper_actor() {
    let mut e = ProvenanceEntry::genesis("post-1", "alice");
    e.actor = "mallory".to_string();
    assert!(!e.verify(), "actor must participate in the hash");
}

#[test]
fn test_provenance_tamper_before_snapshot() {
    let mut e = ProvenanceEntry::genesis("post-1", "alice");
    e.before_snapshot = Some("{\"redacted\":true}".to_string());
    assert!(!e.verify(), "before_snapshot must participate in the hash");
}

#[test]
fn test_provenance_tamper_transformation() {
    let mut e = ProvenanceEntry::genesis("post-1", "alice");
    e.transformation = Some("evil-rewrite".to_string());
    assert!(!e.verify(), "transformation must participate in the hash");
}

Plus a round-trip test: build genesis → chain → chain → chain;
assert each entry verifies; mutate each entry in turn and assert
the mutation breaks verify().

Acceptance

• Three new unit tests as above, all green after V-L2-C1
• One end-to-end round-trip test exercising a 4-entry chain
• Mutation matrix test (each field × each entry) fails after
  every single mutation