Harden candidate-family heuristics: observation_coverage, test_visibility, dependency_drift

[ProtocolWarden]

Context

The decision engine currently has three active candidate families gated by _DEFAULT_ALLOWED_FAMILIES. Each family's rule has configurable thresholds (min_consecutive_runs, pattern keys). After observing real emit patterns, the per-family heuristics need a review pass.

Goal

Review and document the rationale for each family's thresholds. Identify suppression patterns that indicate thresholds are too aggressive or too loose.

Families in Scope

• observation_coverage — fires when a signal is persistently unavailable
• test_visibility — fires when test status is persistently unknown
• dependency_drift — fires when drift is persistently detected

Acceptance Criteria

• Each family has a documented threshold rationale in tuning docs or inline comments
• Suppression reasons in artifacts are reviewed; family_deferred_initial_gating items have a path to promotion
• hotspot_concentration and todo_accumulation promotion criteria are documented

Labels

autonomy, decision-engine, tuning