Graph data-model integrity: unresolved-reference diagnostics, deployment history, tag validity intervals

[toddysm]

Part of #168 (gap analysis: data-model integrity).

Problems (from the #168 gap analysis)

1. Unresolved-reference diagnostics — the indexer MERGEs missing occurrences instead of flagging dangling cross-record references; Supply-chain graph: define file-backed artifact discovery and lineage requirements #168 requires actionable diagnostics rather than silently creating an incomplete graph.
2. Deployment history — RUNS is keyed only by (deployment, occurrence), so deployment-state changes can overwrite history; Supply-chain graph: define file-backed artifact discovery and lineage requirements #168 requires preserving history (like POINTED_TO/REFERS_TO).
3. Tag validity intervals — POINTED_TO stores observedAt but no derivable validity interval.

Acceptance criteria

• Indexer emits diagnostics for unresolved references (opt-in strict mode acceptable).
• Deployment events are append-only / history-preserving.
• Tag→digest relationships expose a validity interval when derivable.
• Tests + docs updated.