Function detect_identity_fork_multi 

pub fn detect_identity_fork_multi(
    old_baselines: &BTreeMap<String, String>,
    new_identities: &BTreeMap<String, String>,
    has_history: bool,
    acknowledged: bool,
) -> Option<ValidationError>

The per-repository generalization of detect_identity_fork for multi-repository policies (plan B5). The unit of identity is the (repo_key, identity-under-that-repo's-identityDefaults) pair, so:

• old_baselines maps each repo_key the OLD object had a resolved identity for (from status.resolved.repositories, with the top-level identity as the single-repo fallback) to its username@hostname;
• new_identities maps each repo_key of the NEW spec’s repository set to the freshly-resolved username@hostname under that repository’s defaults;
• the edit forks iff any repo_key present in BOTH maps resolves differently (the error names that repository). An ADDED repository (in new only) has no history to orphan — no fork. A REMOVED repository (in old only) is not a fork either (the caller surfaces it as a warning).

use std::collections::BTreeMap;
use kopiur_api::error::ValidationError;
use kopiur_api::validate::detect_identity_fork_multi;

let old: BTreeMap<String, String> = [
    ("Repository/billing/a".to_string(), "pg@east".to_string()),
    ("Repository/billing/b".to_string(), "pg@west".to_string()),
].into();
let mut new = old.clone();

// Unchanged → allowed.
assert!(detect_identity_fork_multi(&old, &new, true, false).is_none());

// Repo b re-resolves differently → fork naming b.
new.insert("Repository/billing/b".to_string(), "pg@flipped".to_string());
assert!(matches!(
    detect_identity_fork_multi(&old, &new, true, false),
    Some(ValidationError::IdentityWouldForkInRepository { repo, .. })
        if repo == "Repository/billing/b"
));
// No history / acknowledged → allowed.
assert!(detect_identity_fork_multi(&old, &new, false, false).is_none());
assert!(detect_identity_fork_multi(&old, &new, true, true).is_none());

// An ADDED repository (no old baseline) never forks.
new = old.clone();
new.insert("ClusterRepository/offsite".to_string(), "pg@east".to_string());
assert!(detect_identity_fork_multi(&old, &new, true, false).is_none());